Add OEIS module

## Queries the **Online Encyclopedia of Integer Sequences** for interesting number sequences.

#### Sequence by example:
```
~sequence 1 2 3 4 5
```

#### Sequence by ID number
```
~sequence a27
```

#### Sequence by keywords
```
~sequence positive integers
```

In all cases the following is returned:
```
[oeis] A000027: "The positive integers." - (1, 2, 3, 4, 5, 6, 7, 8, ...) https://oeis.org/A000027
```

modules/oeis/config.json

@@ -0,0 +1,3 @@
+{
+    "outputPrefix": "\u00033oeis\u000f"
+}

modules/oeis/oeis.js

@@ -0,0 +1,114 @@
+/**
+ * Module Name: oeis
+ * Description: Interacts with the Online Encyclopedia of Integer Sequences
+ *              API to provide integer sequence lookups and related information
+ */
+
+var rp = require('request-promise-native');
+
+var OEIS = function(dbot) {
+    this.apiRoot = 'https://oeis.org/search';
+    this.webRoot = 'https://oeis.org/';
+    
+    this.internalAPI = {
+        'parseResult': result => {
+            return {
+                a: result.number,
+                a6: 'A' + result.number.toString().padStart(6, '0'),
+                name: result.name.slice(0, result.name.indexOf('.') + 1),
+                sample: result.data.split(',').slice(0, 8),
+                more: result.data.length > 8,
+                url: this.webRoot + 'A' + result.number.toString().padStart(6, '0')
+            };
+        },
+        
+        'outputError': (event, e) => {
+            switch(e) {
+                case 'no-results': event.reply(dbot.t('sequence_not_found')); return;
+            }
+            
+            console.log(e);
+            event.reply(dbot.t('oeis_error'));
+        }
+    };
+    
+    this.api = {
+        'lookupSequenceByKeywords': async keywords => {
+            var body = await rp({
+                url: this.apiRoot,
+                qs: {
+                    fmt: 'json',
+                    q: '"' + keywords + '"'
+                },
+                json: true
+            });
+            
+            if(!body.results) throw 'no-results';
+            return this.internalAPI.parseResult(body.results[0]);
+        },
+        
+        'lookupSequenceByExample': async (terms, inOrder) => {
+            var joinChar = inOrder ? ',' : ' ';
+            var body = await rp({
+                url: this.apiRoot,
+                qs: {
+                    fmt: 'json',
+                    q: terms.join(joinChar)
+                },
+                json: true
+            });
+            
+            if(!body.results) throw 'no-results';
+            return this.internalAPI.parseResult(body.results[0]);
+        },
+        
+        'lookupSequenceByA': async a => {
+            var body = await rp({
+                url: this.apiRoot,
+                qs: {
+                    fmt: 'json',
+                    q: 'A' + a.toString().padStart(6, '0')
+                },
+                json: true
+            });
+            
+            if(!body.results) throw 'no-results';
+            return this.internalAPI.parseResult(body.results[0]);
+        }
+    };
+    
+    this.commands = {
+        '~sequence': async event => {
+            var result;
+            
+            try {
+                if (event.input[1]) {
+                    // digit sequence
+                    var terms = event.params.slice(1);
+                    result = await this.api.lookupSequenceByExample(terms, true);
+                } else if (event.input[2]) {
+                    // A-number
+                    var a = event.input[2];
+                    result = await this.api.lookupSequenceByA(a);
+                } else {
+                    // Keyword search
+                    var keywords = event.params.slice(1).join(' ');
+                    result = await this.api.lookupSequenceByKeywords(keywords);
+                }
+            }
+            catch(e) {
+                this.internalAPI.outputError(event, e);
+                return;
+            }
+            
+            result.sample = result.sample.join(', ');
+            if (result.more) result.sample += ', ...';
+            
+            event.reply(dbot.t('output_sequence', result));
+        }
+    }
+    
+    this.commands['~sequence'].regex = [/^sequence ((?:-?\d\s?)+)|A((?:\d)+)|([\D\s]*)$/i, 4];
+}
+
+exports.fetch = dbot => new OEIS(dbot);

modules/oeis/strings.json

@@ -0,0 +1,11 @@
+{
+    "output_sequence": {
+        "en": "{a6}: \"{name}\" - ({sample}) {url}"
+    },
+    "sequence_not_found": {
+        "en": "The sequence could not be identified."
+    },
+    "oeis_error": {
+        "en": "Error communicating with the OEIS database."
+    }
+}