Add 'dark_text' option for solarized.vim

doc/airline-themes.txt

@@ -164,7 +164,13 @@ look more like classic powerline in normal mode. To enable it:
 >
     let g:airline_solarized_normal_green = 1
 <
+*g:airline_solarized_dark_text*
 
+Turns the text color of the outer-most sections of the statusline to be dark. 
+To enable it:
+>
+    let g:airline_solarized_dark_text = 1
+<
 *g:airline_solarized_dark_inactive_border*
 
 Changes inactive window panes to have a dark bottom border instead