make :AirlineTheme a getter/setter

doc/airline.txt

@@ -154,7 +154,7 @@ example that you could add to your vimrc:
 COMMANDS                                             *airline-commands*
 >
 :AirlineTheme {theme-name}                           *:AirlineTheme*
-  Changes the current airline theme.
+  Displayes or changes the current theme.
 
 ==============================================================================
 CONTRIBUTIONS                                        *airline-contributions*

plugin/airline.vim

@@ -55,7 +55,13 @@ function! s:get_airline_themes(a, l, p)
   let files = split(globpath(&rtp, 'autoload/airline/themes/'.a:a.'*'), "\n")
   return map(files, 'fnamemodify(v:val, ":t:r")')
 endfunction
-command! -nargs=1 -complete=customlist,<sid>get_airline_themes AirlineTheme call airline#load_theme(<f-args>)
+function! s:airline_theme(...)
+  if a:0
+    call airline#load_theme(a:1)
+  endif
+  echo g:airline_theme
+endfunction
+command! -nargs=? -complete=customlist,<sid>get_airline_themes AirlineTheme call <sid>airline_theme(<f-args>)
 
 augroup airline
   au!