Vim: rename some plugins

I decided that I want the directory names of my Vim plugins to match the
names of their repositories. I think this will make it easier for me to
understand exactly which plugins I am using. In this commit I rename a
few of them that I found that were not named after their repositories.
Vim: remove splitjoin plugin

This plugin seemed like a good idea, but I don't think I used it even
once. Removing it to clear out the clutter.