[RFC] Add optional third tab character in "listchars"

This pull request adds a new feature: a third (optional) character for the "tab" suboption of the "listchars" option. It enables the user to specify a third character for tab, which in turn enables vim to display tabs as '<-->' or '--->' (if the tab takes four spaces).

See issue: https://github.com/neovim/neovim/issues/820

Update to the documentation: https://github.com/pragmb/docs/commit/e62dbf097e8b3590fd704fb119f9b86d2e0fbac0