Add support for flattened and NeoSolarized themes

These are forks/alternate versions of Solarized, without many of the
dynamic elements or the extensive terminal support.

[flattened](https://github.com/romainl/flattened) is a basic, totally
static version of Solarized with only ansi and GUI support.

[NeoSolarized](https://github.com/iCyMind/NeoSolarized) is a version
focusing on true/24-bit color support and NeoVim support, with limited
dynamic options.

[solarized8](https://github.com/lifepillar/vim-solarized8) is a version
focusing on vim/neovim true color support and limited dynamics. It is
already supported (`match()` matches it), but it is at least worth
noting.

Many users, including myself, use these themes over the original
Solarized for performance or technical reasons. Many users believe they
are incompatible with airline because automatic theming no longer
functions. While they can manually override the theme, most people seem
to miss this option in the documentation and give up.

It would be a nice quality of live improvement if airline detected these
themes and enabled its own Solarized theme automatically to match.

autoload/airline/init.vim

@@ -59,6 +59,8 @@ function! airline#init#bootstrap()
         \ 'wombat': 'wombat',
         \ 'zenburn': 'zenburn',
         \ 'solarized': 'solarized',
+        \ 'flattened': 'solarized',
+        \ '\CNeoSolarized': 'solarized',
         \ }, 'keep')
 
   call s:check_defined('g:airline_symbols', {})