Use polylabel for tag component polygon labels

[oscarlorentzon]

Currently it is only possible to set the vertex index for labels on both polygons and rectangles. For polygons it makes much more sense to put the label or icon in a representative point or visual center inside the polygon than at a vertex.

This can be implemented with the help of https://github.com/mapbox/polylabel.

• Add support for configuring the outline tag with a visual center label / icon.
• Use polylabel to determine the visual center.
• Render the label / icon at the visual center.

[oscarlorentzon]

Related to #126