add wordcount extension

resolves #761

autoload/airline/extensions.vim

@@ -206,6 +206,10 @@ function! airline#extensions#load()
     call airline#extensions#whitespace#init(s:ext)
   endif
 
+  if get(g:, 'airline#extensions#wordcount#enabled', 1)
+    call airline#extensions#wordcount#init(s:ext)
+  endif
+
   if get(g:, 'airline#extensions#tabline#enabled', 0)
     call airline#extensions#tabline#init(s:ext)
   endif

autoload/airline/extensions/wordcount.vim

@@ -0,0 +1,32 @@
+" MIT License. Copyright (c) 2013-2015 Bailey Ling.
+" vim: et ts=2 sts=2 sw=2
+
+let g:airline#extensions#wordcount#filetypes = '\vhelp|markdown|rst|org'
+
+" http://stackoverflow.com/questions/114431/fast-word-count-function-in-vim
+function! airline#extensions#wordcount#count()
+  let old_status = v:statusmsg
+  let position = getpos(".")
+  exe "silent normal g\<c-g>"
+  let cnt = 0
+  let stat = v:statusmsg
+  if stat != '--No lines in buffer--'
+    let cnt = str2nr(split(v:statusmsg)[11])
+  end
+  call setpos('.', position)
+  let v:statusmsg = old_status
+  return cnt
+endfunction
+
+function! airline#extensions#wordcount#apply(...)
+  if &ft =~ g:airline#extensions#wordcount#filetypes
+    let spc = g:airline_symbols.space
+    call airline#extensions#prepend_to_section('z',
+          \ '%{airline#extensions#wordcount#count()}' . spc . 'words' . spc . g:airline_right_alt_sep .spc)
+  endif
+endfunction
+
+function! airline#extensions#wordcount#init(ext)
+  call a:ext.add_statusline_func('airline#extensions#wordcount#apply')
+endfunction
+

doc/airline.txt

@@ -401,6 +401,15 @@ eclim <https://eclim.org>
   |airline-syntastic| extension. >
   let g:airline#extensions#eclim#enabled = 1
 
+-------------------------------------                    *airline-wordcount*
+* enable/disable word counting. >
+  let g:airline#extensions#wordcount#enabled = 1
+<
+* regex of filetypes to enable word counting. >
+  " the default value matches filetypes typically used for documentation
+  " such as markdown and help files.
+  let g:airline#extensions#wordcount#filetypes = ...
+<
 -------------------------------------                   *airline-whitespace*
 * enable/disable detection of whitespace errors. >
   let g:airline#extensions#whitespace#enabled = 1