Allow users to define custom before/after callbacks (#3)

README.md

@@ -26,6 +26,31 @@ Configuration
 - `g:goyo_margin_top` (default: 4)
 - `g:goyo_margin_bottom` (default: 4)
 - `g:goyo_linenr` (default: 0)
+- `g:goyo_callbacks` ([before_funcref, after_funcref])
+
+### Callbacks
+
+By default, [vim-airline](https://github.com/bling/vim-airline),
+[vim-powerline](https://github.com/Lokaltog/vim-powerline),
+[powerline](https://github.com/Lokaltog/powerline), and
+[vim-gitgutter](https://github.com/airblade/vim-gitgutter) are temporarily
+disabled while in Goyo mode.
+
+If you have other plugins that you want to disable/enable, or if you want to
+change the default settings of Goyo window, you can define before and after
+callbacks as follows in your .vimrc.
+
+```vim
+function! s:goyo_before()
+  "
+endfunction
+
+function! s:goyo_after()
+  "
+endfunction
+
+let g:goyo_callbacks = [function('s:goyo_before'), function('s:goyo_after')]
+```
 
 Inspiration
 -----------