Add base16_shell theme

[mshkrebtan]

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.

[chrisbra]

please squash and add documentation.

[mshkrebtan]

@chrisbra Please review! :)

[dawikur]

Don't you think those let s:gui_* = #RRGGBB values will differ for each scheme? For cterm this is fine as this uses terminal color palette, but for gui version it uses its own colors instead of scheme.

Also how you define palettes (s:N1, s:N2, s:I3, etc) does not allow scheme to specify its own colors.

I don't think this commit is the ultimate solution for base16 schemes.

[mshkrebtan]

@dawikur I left the GUI-related part untouched. And you are right, the gui values are hardcoded and match only one exact theme. This vim-airline theme can only match command-line vim.

Is it actually possible to integrate Base16 Shell with GUI vim?

[dawikur]

Well, base16-vim/shell defines colors also in #RRGGBB, unfortunately both of them unsets those variables at the end. That is why I was asking about template for base16 colors for airline (see #64 )