Update vim config

- remove powerline plugin (statusline plugin)
- add airline plugin (statusline plugin)
- add tagline plugin (ctags tag browsing plugin)
- organize things a bit
- change mappings:
  - 'g' now runs `tig`
  - 'q' in help files closes the window
  - 't' opens tagbar and focuses it
- remove some (now unneeded) config around highlighting the current
line/column