Change to dark terminal theme

Config files for vim and tmux needs to be updated to support solarized
dark as terminal theme.
Add alias for `git diff --cached`
Use Base16 color scheme instead of Solarized

Remove references to Solarized, including a function to toggle between
dark and light background color.

More information:
- https://github.com/chriskempson/base16
- https://github.com/chriskempson/base16-vim
Match color definitions to Base16

Some colors, such as the statusline, were defined according to the
Solarized color scheme. Since Base16 is now in use, the colors need to
be updated to match Base16 instead.
[tmux] Use Base16 color scheme instead of Solarized

Some of the statusbar colors need to be updated to match the color
scheme of Base16. Also remove some of the no longer relevant comments.
[vim] Remove unused plugins
[vim] Group syntax related plugins together
[vim] Change comment referring to Solarized
[vim] Use fugitive.vim for Git awesomeness

fugitive.vim is a Git wrapper that makes it almost too easy to work with
Git from within vim. It removes the need to exit vim to commit files and
do other Git related tasks.

More information:
- https://github.com/tpope/vim-fugitive
[vim] Add plugin for text filtering and alignment

Including mappings to quickly align equal signs and colons.

More information:
- https://github.com/godlygeek/tabular
- http://vimcasts.org/episodes/aligning-text-with-tabular-vim/
[vim] Add plugin to enhance netwr

vingear.vim enhances netwr, the built in directory browser.

More information:
- https://github.com/tpope/vim-vinegar
[vim] Use a simpler statusline

The statusline now contains the following:
- Current file, including file path
- Flags
- Git status (from fugitive.vim)
- Syntax errors (from Syntastic)
- Current line number and total lines
[vim] Remove custom highlight of cursorline

It is not necessary to set the colors of the cursorline since cusorline
is disabled.
[vim] Set Syntastic options

- Use RuboCop for Ruby files and use the Rails cops.
`syntastic_ruby_checkers = ['rubocop']`
`syntastic_ruby_rubocop_args = '-R'`

- Stick detected errors into the location list.
`syntastic_always_populate_loc_list = 1`

- Mark syntax errors using the :sign interface.
`syntastic_enable_signs = 1`

- Set the format of the statusbar details.
`syntastic_stl_format = 'SYNTAX(%E{err: %e}%B{ }%W{warn: %w})'`
[vim] Visual changes to statusline

Remove the brackets (`[]`) around flags, Git status and Syntastic
messages. The flags are now separated by spaces.

Git status, Syntastic messages and line numbers are formatted like
functions, e.g. GIT(branch) and LINE(10/77). Also, Syntastic messages
are highlighted in red instead of the default statusline color.
[vim] Open documentation in Dash

Set investigate to open results in Dash
[vim] Clean up LESS mapping

Remove duplicate LESS mapping. Also remove hard coded comma, which was
the previous leader.
[vim] Add mappings to increase/decrease numbers

Increasing or decreasing the number currently below the cursor or on the
same line is by default done by pressing C-a or C-x. C-a interferes with
tmux, where it is the prefix key. These alternative mappings makes it
possible to increase and decrease numbers using <leader>-a and
<leader>-x.

More information:
- http://vim.wikia.com/wiki/Increasing_or_decreasing_numbers
[vim] Enable persistent undo

Persistent undo saves the undo history even after a buffer is closed.
The undo history files are saved to the $HOME/.vimundo directory.

More information:
- http://choorucode.com/2010/11/27/vim-persistent-undo/
[vim] Improved mappings for buffers

Adds two mappings, <leader>b (open buffer in current window) and
<leader>B (open buffer in a vertial split window), to quickly switch
between open buffers. Unlike the old mapping, the new makes it easy to
switch buffers using Tab.