plugin: mention how to disable certain events

E.g. how to disable the FocusGained event by setting the 'eventignore'
option.

Add some boilerplate to skip the focusgained function handler function, if it should be
ignored. (this should in theory stop setting up the autocommands at all
so might be tiny performance optimization).

closes #2421

doc/airline.txt

@@ -2080,6 +2080,11 @@ Q. Performance is bad
 A. Check the question at the wiki:
    https://github.com/vim-airline/vim-airline/wiki/FAQ#i-have-a-performance-problem
 
+Q. Something strange happens on certain autocommands
+A. Try to disable the autocommand by setting the 'eventignore' option like
+   this: >
+   set eventignore+=FocusGained
+
 Solutions to other common problems can be found in the Wiki:
 <https://github.com/vim-airline/vim-airline/wiki/FAQ>
 

plugin/airline.vim

@@ -75,6 +75,10 @@ function! s:on_window_changed(event)
 endfunction
 
 function! s:on_focus_gained()
+  if &eventignore =~? 'focusgained'
+    return
+  endif
+
   if airline#util#try_focusgained()
     unlet! w:airline_lastmode | :call <sid>airline_refresh(1)
   endif
@@ -148,7 +152,7 @@ function! s:airline_toggle()
       autocmd CursorMoved * call <sid>on_cursor_moved()
 
       autocmd VimResized * call <sid>on_focus_gained()
-      if exists('*timer_start') && exists('*funcref')
+      if exists('*timer_start') && exists('*funcref') && &eventignore !~? 'focusgained'
         " do not trigger FocusGained on startup, it might erase the intro screen (see #1817)
         " needs funcref() (needs 7.4.2137) and timers (7.4.1578)
         let Handler=funcref('<sid>FocusGainedHandler')
@@ -234,7 +238,7 @@ function! s:airline_refresh(...)
 endfunction
 
 function! s:FocusGainedHandler(timer)
-  if exists("s:timer") && a:timer == s:timer && exists('#airline')
+  if exists("s:timer") && a:timer == s:timer && exists('#airline') && &eventignore !~? 'focusgained'
     augroup airline
       au FocusGained * call s:on_focus_gained()
     augroup END