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__))}]'
Merge pull request #441 from blueyed/tagbar_once_per_second

tagbar: only lookup tag once every second
Merge branch 'tabline-tab_nr_type-as-string'
reset current buffer when one gets deleted. resolves #399
Merge pull request #404 from blueyed/solarized-inactive-like-vertsplit

solarized: use VertSplit bg for inactive statusline
Merge branch 'master' of git://github.com/bling/vim-airline