[vim] Add `toggle limits` functionality

Add the possibility to toggle the background color of certain columns to
indicate specific limits (e.g.: columns 51 & 73 for git commit messages
http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html).
[shell] Use auto list TAB completion

Replace the use of TAB for cycling through all possible completions
(as it can be annoying) with the ability to auto list completions.
[vim] Use Solarized colorscheme for MacVim GUI

- Add Solarized colorscheme as a git submodule:
  https://github.com/altercation/vim-colors-solarized.
- Add a `gvimrc` file for GUI-specific settings:
  http://vimdoc.sourceforge.net/htmldoc/gui.html.
[vim] Add xoria256 colorscheme as a git submodule