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

autoload/airline/extensions.vim

@@ -23,12 +23,15 @@ endfunction
 let s:script_path = tolower(resolve(expand('<sfile>:p:h')))
 
 let s: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()}'],
       \ }
 
 let s:filetype_regex_overrides = {}
@@ -58,18 +61,13 @@ function! airline#extensions#apply_left_override(section1, section2)
 endfunction
 
 function! airline#extensions#apply(...)
+  let filetype_overrides = get(s:, 'filetype_overrides', {})
+  call extend(filetype_overrides, get(g:, 'airline_filetype_overrides', {}), 'force')
 
   if s:is_excluded_window()
     return -1
   endif
 
-  if &buftype == 'help'
-    call airline#extensions#apply_left_override('Help', '%f')
-    let w:airline_section_x = ''
-    let w:airline_section_y = ''
-    let w:airline_render_right = 1
-  endif
-
   if &buftype == 'terminal'
     let w:airline_section_x = ''
     let w:airline_section_y = ''
@@ -81,11 +79,20 @@ function! airline#extensions#apply(...)
     let w:airline_section_c = bufname(winbufnr(winnr()))
   endif
 
-  if has_key(s:filetype_overrides, &ft)
+  if has_key(filetype_overrides, &ft) &&
-    let args = s:filetype_overrides[&ft]
+        \ ((&filetype == 'help' && &buftype == 'help') || &filetype !~ 'help')
+    " for help files only override it, if the buftype is also of type 'help',
+    " else it would trigger when editing Vim help files
+    let args = filetype_overrides[&ft]
     call airline#extensions#apply_left_override(args[0], args[1])
   endif
 
+  if &buftype == 'help'
+    let w:airline_section_x = ''
+    let w:airline_section_y = ''
+    let w:airline_render_right = 1
+  endif
+
   for item in items(s:filetype_regex_overrides)
     if match(&ft, item[0]) >= 0
       call airline#extensions#apply_left_override(item[1][0], item[1][1])
@@ -213,14 +220,9 @@ function! airline#extensions#load()
   endif
 
   if exists(':VimShell')
-    let s:filetype_overrides['vimshell'] = ['vimshell','%{vimshell#get_status_string()}']
     let s:filetype_regex_overrides['^int-'] = ['vimshell','%{substitute(&ft, "int-", "", "")}']
   endif
 
-  if exists(':Defx')
-    let s:filetype_overrides['defx'] = ['defx', '%{b:defx.paths[0]}']
-  endif
-
   if get(g:, 'airline#extensions#branch#enabled', 1) && (
           \ airline#util#has_fugitive() ||
           \ airline#util#has_lawrencium() ||

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