Add an option to truncate text on left instead of right

[nraw]

When the space in the statusline gets low, the truncation will happen so that the right most characters are visible, but if the let g:bufferline_fixed_index = 0 then you don't get to see the active buffer anymore.

Ideally the truncation would allow to leave the left most sides visible for the longest.

Or alternatively truncate the active buffer as the last resort.