[Tilesathome] Requesting rendering of boundary=national_park

[Ævar Arnfjörð Bjarmason]

Tag:boundary_national park is currently not rendered by t at h. I think
it should be rendered like Tag:boundary=administrative except with a
green dotted line instead of a red one. At least that's how JOSM
renders it and national parks can be thought of as "green borders".

Could someone familiar with osmarender please implement this? I've
been adding some national parks and it would be much appreciated to
get them on one of the maps.