Run custom ruby scripts as post-command hooks

Inspired by `git-hooks`, I've added the concept of callback hooks so that **timetrap** commands can be extended by the end user or through third-party gems.

After any command is finished, if there is a Ruby file at `~/.timetrap/hooks/#{current_sheet}/#{command}.rb`, it will be required before the process exits.

I needed this functionality to support my [timetrap-hipchat](https://github.com/logankoester/timetrap-hipchat) gem, which notifies a chat room when I am billing time and what I'm working on.

![hipchat](https://raw.githubusercontent.com/logankoester/timetrap-hipchat/master/demo.png)

Would love to see this merged in! Let's discuss :smile_cat: