Add ERB support for yml config files, cache result of raw_content

[SidOfc]

Added ERB support in ~/.teamocil/*.yml files

This change allows you to write dynamic ruby code within your yml config files.
This means you can base all your commands in your config (to be executed on panes / windows) on a path that is relative to the current directory, using root: <%= Dir.pwd %> for instance.

cache result of raw_content method in lib/layout.rb

This change is simply a side-effect of the above addition, I noticed the method is called multiple times, re-reading the yml file, since I added ERB support I figured it would be nice to cache the result after fetching it :)

Hope you will accept the change, I'll be installing my commit locally for now, cheers for this command line tool, it's fantastic!

[SidOfc]

There seems to be an encoding error coming from the specs, trying to solve it now but I'm probably going to have to look at this some more tomorrow.