This allows inclusion of the usual standardSubstitute vars within factoids.
There is no config to disable this, since it is possible to escape the substitutions
by simply doubling the dollar signs, as per the python documentation:
http://docs.python.org/library/string.html#template-strings
Thus, if you want a factoid to output a literal "$channel", for example, all
you'd need to do is use "$$channel" in your factoid text, which will come out
as "$channel" when said by the bot.
Also added tests for this new behavior.
976ad82d69