make warning a real section. references #136.

autoload/airline.vim

@@ -109,7 +109,7 @@ function! airline#get_statusline(winnr, active)
 
     if a:active
       let sl.='%(%#Al2_to_warningmsg#'.g:airline_right_sep
-      let sl.='%#warningmsg#'.s:getwinvar(a:winnr, 'airline_section_warning', '').'%)'
+      let sl.='%#warningmsg#'.s:get_section(a:winnr, 'warning', '', '').'%)'
     endif
   endif
   return sl

doc/airline.txt

@@ -139,7 +139,6 @@ CUSTOMIZATION                                        *airline-customization*
 
 The following are some unicode symbols for customizing the left/right
 separators, as well as the powerline font glyths.
-
 >
   " unicode symbols
   let g:airline_left_sep = '»'
@@ -175,10 +174,12 @@ separators, as well as the powerline font glyths.
 <
 
 For more intricate customizations, you can replace the predefined sections
-with the usual statusline syntax.
-
+with the usual statusline syntax. Note that many of these settings can also be
+controlled with |airline-configuration| variables, which is useful for
+sections which by default host more than one extension.
 >
-  " here are the the default values
+  variable names                default contents
+  ----------------------------------------------------------------------------
   let g:airline_section_a       (the mode/paste indicator)
   let g:airline_section_b       (the fugitive/lawrencium branch indicator)
   let g:airline_section_c       (bufferline or filename)

plugin/airline.vim

@@ -63,6 +63,7 @@ call s:check_defined('g:airline_section_gutter', '')
 call s:check_defined('g:airline_section_x', "%{strlen(&filetype)>0?&filetype:''}")
 call s:check_defined('g:airline_section_y', "%{strlen(&fenc)>0?&fenc:''}%{strlen(&ff)>0?'['.&ff.']':''}")
 call s:check_defined('g:airline_section_z', '%3p%% '.g:airline_linecolumn_prefix.'%3l:%3c')
+call s:check_defined('g:airline_section_warning', '')
 
 let s:airline_initialized = 0
 function! s:on_window_changed()