Support including all buffers in the tabline, à la buftabline

[ELLIOTTCABLE]

Exactly what the title says. See vim-buftabline.

Basically, I'd like Lightline to conform a little more to a classic-vim workflow, as opposed to a “file-per-vimtab” workflow. (=

Alternatively, simply integrate directly with buftabline, if installed? Override its styles and such? I'm not sure if that's possible, I suspect buftabline assumes it has full control of the tabline's content.

[itchyny]

See #155 (comment).

[ELLIOTTCABLE]

@itchyny That does indeed do exactly what I wanted! Thank you!