wordcount: allow to disable vimtex wordcount()

While Commit e542f5e introduced a nice little feature for TeX files to
accurately count words, this unfortunately has the drawback of being
possibly slow, especially since the statusline is often re-evaluated.

Therefore disable this feature by default. You can enable it using:

    :let g:airline#extensions#vimtex#wordcount = 1

fixes #2461

autoload/airline/extensions/wordcount.vim

@@ -9,7 +9,7 @@ if exists('*wordcount')
     if get(g:, 'actual_curbuf', '') != bufnr('')
       return
     endif
-    if &filetype ==# 'tex' && exists('b:vimtex')
+    if &filetype ==# 'tex' && exists('b:vimtex') && get(g:, 'airline#extensions#vimtex#wordcount', 0)
       " We're in a TeX file and vimtex is a plugin, so use vimtex's wordcount...
       return vimtex#misc#wordcount()
     else

doc/airline.txt

@@ -1507,6 +1507,11 @@ State indicators:
 * viewer is opened >
   let g:airline#extensions#vimtex#viewer = "v"
 
+* use vimtex specific wordcount function
+  for TeX buffers Note: this more accurate
+  but may slow down Vim) >
+  let g:airline#extensions#vimtex#wordcount = 1 
+
 -------------------------------------                   *airline-virtualenv*
 virtualenv <https://github.com/jmcantrell/vim-virtualenv>