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!
There's documentation about how to write scripts in docs/scripting.md. I'm suggesting that might be a good place to talk about `robot.logger` in your own scripts.