add functionality of logging to file

Hi guys,

I am feeling it is quite useful to make logs be persisted to disk files rather that only print to stdout. Especially when I also want to use robot.logger to log some script layer information, I would like to have it be kept on file and with hubot internal logging information.

So the easiest way of achieving this is simply use a global variable HUBOT_LOG_FILE, if it is specified, then we will construct a Log instance with a file stream.

Hope this make sense to you, Thanks a lot!
I'd move this up to the top of the section, and include some examples about it.

We can also link to the library it's using, ie https://www.npmjs.org/package/log