solarized: add support for g:solarized_base16 by @cuviper

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.

doc/airline-themes.txt

@@ -23,7 +23,7 @@ conjuction with |vim-airline|
 ==============================================================================
 FEATURES                                              *airline-themes-list*
 
-Currently this repository contains the following themes: >
+Currently this repository contains the following themes:
 
     * badwolf
     * base16
@@ -90,7 +90,7 @@ Currently this repository contains the following themes: >
     * silver
     * simple
     * sol
-    * solarized
+    * solarized (|airline-theme-solarized|)
     * term
     * tomorrow
     * ubaryd
@@ -98,11 +98,22 @@ Currently this repository contains the following themes: >
     * wombat
     * xtermlight
     * zenburn
-<
+
 ==============================================================================
 NAME                                         *airline-themes-configuration*
 
-(None yet)
+                                                  |airline-theme-solarized|
+*g:solarized_base16*
+
+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.  To enable it:
+>
+    let g:solarized_base16 = 1
+>
+See also https://github.com/blueyed/vim-colors-solarized/commit/92f2f994.
 
 ==============================================================================
 CONTRIBUTIONS                                *airline-themes-contributions*