Add autocomplete script to the gem

[bunnymatic]

Attempt to address #46

• add post install message
• add autocomplete script from @alexeyshockov

I tried this build locally and added it looks good. The place it gets a little weird is when you switch projects and rbenv (or rvm) doesn't have lunchy installed in that gemset. Or if you move to a project directory (with a ruby version manager) that has an older version of lunchy. At that point, the gem which lunchy will return a path that points to the old version and the completion is not available.

I don't know if that warrants installation instructions (like put this in your system gem set) or if it just rights itself over time because overtime, if you have multiple versions of lunchy installed, they'll all have the bash completion and things will just work.

[eddiezane]

Thanks for digging into this one!