extensions: Allow customization of filetypes

Define the variable :let g:airline_filetype_overrides to allow for
specific customization how a specific filetype will be displayed.

For e.g. a German display, you can use: >

    :let g:airline_filetype_overrides = {'help': ['Hilfe', ':%f']}
<

This defines how section a and b will look like for a window with that
specific filetype.

Closes #1888

doc/airline.txt

@@ -203,12 +203,27 @@ values):
   statusline modified >
   let g:airline_exclude_filetypes = [] " see source for current list
 <
+* define the set of names to be displayed instead of a specific filetypes (for
+  section a and b): >
+
+  let g:airline_filetype_overrides = {
+      \ 'defx':  ['defx', '%{b:defx.paths[0]}'],
+      \ 'gundo': [ 'Gundo', '' ],
+      \ 'help':  [ 'Help', '%f' ],
+      \ 'minibufexpl': [ 'MiniBufExplorer', '' ],
+      \ 'nerdtree': [ get(g:, 'NERDTreeStatusline', 'NERD'), '' ],
+      \ 'startify': [ 'startify', '' ],
+      \ 'vim-plug': [ 'Plugins', '' ],
+      \ 'vimfiler': [ 'vimfiler', '%{vimfiler#get_status_string()}' ],
+      \ 'vimshell': ['vimshell','%{vimshell#get_status_string()}'],
+      \ }
+<
 * defines whether the preview window should be excluded from have its window
   statusline modified (may help with plugins which use the preview window
   heavily) >
   let g:airline_exclude_preview = 0
 <
-* disable the Airline customization for selective windows (this is a
+* disable the Airline customization for selected windows (this is a
   window-local variable so you can disable it per-window) >
   let w:airline_disabled = 1