tabline: add support for tab_nr_type as string

'__tabnr__' gets replaced with the tab number. This allows for custom
configurations, where you want to have both the tab number and the
number of windows at the same time, e.g.:

    let g:airline#extensions#tabline#tab_nr_type = '[__tabnr__.%{len(tabpagebuflist(__tabnr__))}]'