Update email config

- allow sending messages with no body
- cache headers in folders instead of flat files
Update vim config and plugins

- 'q' to quit in help windows and the quickfix window
- highlight the current line in the quickfix window, the tagbar plugin,
  and git/fugitive windows
- default to literal text search with ag (faster, no need to escape most
  things)
Handle symlinks when setting wallpaper
Update XMonad config

- 24 hour time (with Mod-Shift-t)
- use the bee screensaver when locking the screen (with Mod-Shift-l)