extract buffer name logic into a formatter.

hold off on deprecation warnings for now...

doc/airline.txt

@@ -305,16 +305,6 @@ virtualenv <https://github.com/jmcantrell/vim-virtualenv>
 * enable/disable displaying buffers with a single tab. >
   let g:airline#extensions#tabline#show_buffers = 1
 <
-* defines a function for how to format the file name. >
-  " the default renders /foo/bar/file.txt => /f/b/file.txt
-  let g:airline#extensions#tabline#fnamefunc
-
-  " here's a simple example to show only the file name:
-  function! MyFileFormat(file)
-    return fnamemodify(a:file, ':t')
-  endfunction
-  let g:airline#extensions#tabline#fnamefunc = 'MyFileFormat'
-<
 * configure filename match rules to exclude from the tabline. >
   let g:airline#extensions#tabline#excludes = []
 <
@@ -322,12 +312,25 @@ virtualenv <https://github.com/jmcantrell/vim-virtualenv>
   let g:airline#extensions#tabline#tab_nr_type = 0 " # of splits (default)
   let g:airline#extensions#tabline#tab_nr_type = 1 " tab number
 <
+* defines the name of a formatter for how buffer names are displayed. >
+  let g:airline#extensions#tabline#formatter = 'default'
+
+  " here is how you can define a 'foo' formatter:
+  function! airline#extensions#tabline#formatters#foo(bufnr)
+    return fnamemodify(bufname(a:bufnr), ':t')
+  endfunction
+  let g:airline#extensions#tabline#formatter = 'foo'
+<
 * configure whether buffer numbers should be shown. >
   let g:airline#extensions#tabline#buffer_nr_show = 0
 <
+Note: this is only valid when the formatter is 'default'.
+
 * configure how buffer numbers should be formatted with |printf|. >
   let g:airline#extensions#tabline#buffer_nr_format = '%s: '
 <
+Note: this is only valid when the formatter is 'default'.
+
 * configure the minimum number of buffers needed to show the tabline. >
   let g:airline#extensions#tabline#buffer_min_count = 0
 <