Cleanup .vim directory + minor changes

- Major cleanup of .vim directory, removing things I no longer use.
- Add clear buffer mapping for tmux (<Ctrl-A)-k
- Change behavior of zt so that line currently under cursor goes to top
  of screen
- Add alias to .zshrc for checking to see if currently in vim (i.e. in a
  shell, but from an instance of vim)
- Add --distribute to alias that creates Python virtual environments