Add the ability to send CSS that will render (some selected part of) a <map-feature> as a text label.

[prushforth]

Maybe we'll have to dream up how this might be done in CSS, which I don't think is very 'hackable' at this time. However it would be ideal to be able to label <map-feature>s with their <map-featurecaption>, for example, or possibly other content drawn from <map-properties>.

We should also consider how to apply this technique in the context of MapML sent as vector tiles, where <map-feature>s can span multiple tiles.