Add commentary.vim

This means I don't have to use things like <c-v> to select big blocks of
code to comment out.
Make fugitive.vim the default Git mergetool

fugitive.vim provides an awesome interface for Git merges. I want to use
it to resolve merge conflicts whenever possible. I could set it up to
run through Vim or MacVim. Because I use the latter whenever possible, I
want to use it to open fugitive. This could lead to problems on systems
that don't have MacVim installed.

To prevent these potential problems, though, I can alias `mvim` to `vim`
whenever `which mvim` fails to find something.

This also allows me to set MacVim as the default editor. This is
something tmuxinator likes to see set.