Add a mode for base16-shell's extended color map

Base16 has a Solarized theme with the usual colors, but mapped in the
terminal differently.  The main difference is that the bright colors,
Ansi 9-15, are left the same as their Ansi 1-7 counterparts.  The
remaining solarized colors are mapped into higher indexes by using
Base16 Shell.  Then enable it:

  let g:solarized_base16=1
  colorscheme solarized