base16_shell theme for vim-airline matches an active Base16 Shell
theme.
It should be used in combination with Base16 Shell, which sets the
correct colors in the terminal:
(https://github.com/chriskempson/base16-shell).
The colors used in this theme are within the 21 term colors set by
Base16 Shell. The theme supports 2 options described in the docs.
Ref: https://github.com/blueyed/vim-colors-solarized/commit/92f2f994
This also defines 'g' (gui) colors, which might not be necessary and
cause unnecessary overhead (at least it caused an error when given an
invalid color (RGB without hash symbol), but was added for completeness.
