Compatibility with vim-airline. Restore line numbers after leave Goyo mode

After opening a new buffer in Goyo mode and then leave Goyo mode, the buffer is not displayed in tabline ([vim-airline](https://github.com/vim-airline/vim-airline)). Then if you switch to this buffer, it will be displayed without line numbers.

Neovim 0.2.2 on linux

Start by reproducing the behavior described when opening a new buffer in Goyo mode, leaving that mode, and switching to the buffer with vim-airline enabled. Trace the Goyo mode-exit handling and its interaction with the tabline, then verify that the buffer appears in the tabline and line numbers are restored after leaving Goyo mode.

Written by the indexing model from the issue text.