feat: add fill_hl config option

[folliehiyuki]

Make the colors of the filler space configurable.

From :h syntax.txt, I think we should use TabLineFill instead of TabLine. This might break people's tabline color, so I make the highlight group a config option and naively pass it to the render() function.

							*hl-TabLine*
TabLine		Tab pages line, not active tab page label.
							*hl-TabLineFill*
TabLineFill	Tab pages line, where there are no labels.

[willothy]

This change makes sense to me overall, I don't think the config variable makes sense to pass to render() though. It's already in the global config table, we can just use the global from within render() instead. I'll look over this in more detail tonight though.

[willothy]

lgtm!