Add ability to specify ruby version for rbenv exec

I added an additional flag and parameter to allow a user to choose the Ruby to use during an `rbenv exec` invocation.

My original motivation was, before installing the heroku toolbelt, checking to ensure I had no gem installed for the Ruby I was working in.  Turns out I did not, but I had some other ones that I also wanted to remove without having to track down which projects were using those Ruby versions.

Another nice possibility is being able to bring up an `irb` console (or what have you) without having to navigate through the directory structure in order to switch to that Ruby.

Please let me know if you would like me to make any changes to this pull request or if you would have done something differently.  I do hope you like the new feature and also consider it useful.