This PR adds support for `capital=6` in the place layer, which is used for tagging county seats in the US. A county seat is the capital of a county. In addition, I reorganized the code a bit to be simpler.
As noted in ZeLonewolf/openstreetmap-americana#384, we would like to render county seats along with state and national capitals, as this is typical styling on American-style maps.
Update with also with `capital=3` and `capital=5`