format param chicken-egg

[yhahn]

Looked briefly into adding a format attribute to projects for defining mapnik image output format (e.g. png8). There's a chicken-egg problem atm in that tilelive-vector tries to set format at init time before the map has been loaded -- but the map needs to be loaded to read the parameters to set the format.

Should be fixable upstream, at which point this option can be added sensibly.