processOutput() should handle numbers

[zuzak]

If a command's function returns a Number, the bot won't output it to a channel. It probably should. This arose in #396.

To fix this, processOutput() should be updated to handle passing in numbers.

The best option is perhaps:

• check if the output is a number
• if it is, convert the number to a string
• call the processOutput a second time (i.e. recursively)

Something like this:

if (typeof output === 'number') {
   output = output.toString()
   return processOutput(bot, msg, cmd, output, customFormatter)
}