tabline: add compatibility with nvim >= 0.5.0
tabline: add compatibility with nvim >= 0.5.0

We expected the tabline messages to be arrays of two elements. Neovim
0.5.0 added more arguments. We currently don't do anything with them,
but at least the tabline is fixed, for now.

Partially fixes #69

jeanguyomarch/eovim,jeanguyomarch/eovim,jeanguyomarch/eovim