tabline: allow custom formatter for tabnr_type

closes #1418

autoload/airline/extensions/tabline/formatters/tabnr.vim

@@ -0,0 +1,14 @@
+" MIT License. Copyright (c) 2017 C.Brabandt
+" vim: et ts=2 sts=2 sw=2
+
+scriptencoding utf-8
+
+function! airline#extensions#tabline#formatters#tabnr#format(tab_nr_type, nr)
+  if a:tab_nr_type == 0 " nr of splits
+    return (g:airline_symbols.space).'%{len(tabpagebuflist('.a:nr.'))}'
+  elseif a:tab_nr_type == 1 " tab number
+    return (g:airline_symbols.space).a:nr
+  else "== 2 splits and tab number
+    return (g:airline_symbols.space).a:nr.'.%{len(tabpagebuflist('.a:nr.'))}'
+  endif
+endfunction