Make fileformat output truncatable

closes #1309

autoload/airline/parts.vim

@@ -88,5 +88,12 @@ function! airline#parts#filetype()
 endfunction
 
 function! airline#parts#ffenc()
-  return printf('%s%s%s', &fenc, &l:bomb ? '[BOM]' : '', strlen(&ff) > 0 ? '['.&ff.']' : '')
+  let expected = get(g:, 'airline#parts#ffenc#skip_expected_string', '')
+  let bomb     = &l:bomb ? '[BOM]' : ''
+  let ff       = strlen(&ff) ? '['.&ff.']' : ''
+  if expected is# &fenc.bomb.ff
+    return ''
+  else
+    return &fenc.bomb.ff
+  endif
 endfunction

doc/airline.txt

@@ -300,6 +300,12 @@ configuration values that you can use.
   Try setting this to zero, if you notice bleeding color artifacts >
   let airline#extensions#default#section_use_groupitems = 1
 <
+* configure the fileformat output
+  By default, it will display something like 'utf-8[unix]', however, you can
+  skip displaying it, if the output matches a configured string. To do so,
+  set >
+  let g:airline#parts#ffenc#skip_expected_string='utf-8[unix]'
+<
 -------------------------------------                     *airline-quickfix*
 The quickfix extension is a simple built-in extension which determines
 whether the buffer is a quickfix or location list buffer, and adjusts the