Add scrolling support like buftabs

I just add new rotate style like buftabs and also keep the original compatibility.

`let g:bufferline_rotate=0`
no rotate, no scrolling

`let g:bufferline_rotate=1`
Scrolling with fixed current buffer position, original bufferline behaviour.

`let g:bufferline_rotate=2`
Scrolling without fixed current buffer position, buftabs behaviour.