Tabline: Make specifying formatter more easy

closes #1863

autoload/airline/extensions/tabline/tabs.vim

@@ -118,7 +118,15 @@ function! airline#extensions#tabline#tabs#map_keys()
   noremap <silent> <Plug>AirlineSelectNextTab :<C-U>exe repeat(':tabn\|', v:count1)<cr>
 endfunction
 
-function! airline#extensions#tabline#tabs#tabnr_formatter(nr, i)
+function! airline#extensions#tabline#tabs#tabnr_formatter(nr, i) abort
   let formatter = get(g:, 'airline#extensions#tabline#tabnr_formatter', 'tabnr')
-  return airline#extensions#tabline#formatters#{formatter}#format(a:nr, a:i)
+  try
+    return airline#extensions#formatters#{formatter}#format(a:nr, a:i)
+  catch /^Vim\%((\a\+)\)\=:E117/	" catch E117, unknown function
+    " Function not found
+    return call(formatter, [a:nr, a:i])
+  catch
+    " something went wrong, return an empty string
+    return ""
+  endtry
 endfunction

doc/airline.txt

@@ -809,6 +809,13 @@ with the middle mouse button to delete that buffer.
 
   Note: last option can be used to specify a different formatter for
   displaying the numbers. By default tabline/formatter/tabnr.vim is used
+  The argument of that setting should either be a filename that exists 
+  autoload/airline/extensions/tabline/formatter/ (without .vim extension)
+  and needs to provide a format() function. Alternatively you can use a
+  custom function name, that is defined e.g. in your .vimrc file. In any
+  case, the function needs to take 2 arguments, tab_nr_type and tabpage
+  number.
+  For an example, have a look at the default tabline/formatter/tabnr.vim
 <
 * enable/disable displaying tab number in tabs mode. >
   let g:airline#extensions#tabline#show_tab_nr = 1