extract bufferline into its own file

autoload/airline/extensions/bufferline.vim

@@ -0,0 +1,12 @@
+" MIT license. Copyright (c) 2013 Bailey Ling.
+" vim: ts=2 sts=2 sw=2 fdm=indent
+
+function! airline#extensions#bufferline#init()
+  highlight AlBl_active gui=bold cterm=bold term=bold
+  highlight link AlBl_inactive Al6
+  let g:bufferline_inactive_highlight = 'AlBl_inactive'
+  let g:bufferline_active_highlight = 'AlBl_active'
+  let g:bufferline_active_buffer_left = ''
+  let g:bufferline_active_buffer_right = ''
+  let g:bufferline_separator = ' '
+endfunction