Minor enhancement for tabline

Show index (not buf number) of each buffer.
Add key mappings to switch to a buffer easily using the index.
you would first define it as a script-scope variable, like so:

    let s:buffer_idx_mode_buffers = []

and to expose it for mappings, you would use `<plug>`, like so:

    noremap <unique> <Plug>AirlineSelectTab0 :call <sid>select_tab(0)<cr>

and then you should be able to do something like this in your vimrc:

    nmap <leader>0 <Plug>AirlineSelect0

see [here](http://vim.wikia.com/wiki/Mapping_keys_in_Vim_-_Tutorial_(Part_3))

haven't tried it yet, but should work...