`gf` go-to-file mapping goes-to-gem-source

Everyone knows (or does not know) that gf when the cursor is over a word in vim will try to find a file on the system to navigate to that matches that word or path.

Rails overrides this to be used on constants and to navigate to the file where a constant is defined if the constant name matches the file name (matches in the rails sense).

When you're in the Gemfile, gf on a gem will take you to that gem, just like bundle open. Brilliant!