Uses POSIX shell parameter expansion syntax.
<pragma-> !factadd cookie /me gives a cookie to ${args:-nobody. What a shame}!
<PBot> cookie added to the global channel.
<pragma-> !cookie Bob
* PBot gives a cookie to Bob!
<pragma-> !cookie
* PBot gives a cookie to nobody. What a shame!
<pragma-> !factadd sum /call calc $arg[0]:-1 + $arg[1]:-2
<PBot> sum added to the global channel.
<pragma-> !sum
<PBot> 1 + 2 = 3
<pragma-> !sum 3
<PBot> 3 + 2 = 5
<pragma-> !sum 4 6
<PBot> 4 + 6 = 10
