Fix buffer numbering to start from 1 with buffer_idx_mode
Fix buffer numbering to start from 1 with buffer_idx_mode

275ec4fe63d9776a2c2cbf72b1f371b9e01273b8 broke this so that numbering
started from 0 instead of 1 with
g:airline#extensions#tabline#buffer_idx_mode = 1

Specifically, I overlooked that |index| actually tracked the index of
the current buffer in the buffer list *starting from 1*, whereas I
assumed it tracked the true index (starting from 0).

rchicoli/vim-airline,chrisbra/vim-airline,yzhou61/vim-airline,bling/vim-airline