Add haskell-mode plugin.

Haskell specific configuration resides in joelpet/vim-ft-config.
Configure airline theme and statusbar.

- Use solarized theme to match the global Vim theme.
- Always show statusbar so airline is visible even in single windows.
Configure CtrlP.

- Change mapping for buffer mode to one that can be differentiated from
  file mode.
- Follow symlinks.
- Change number of results in match windows.
- Disable working path mode.
Remove trailing whitespace.