Make dependency on Proj4, Proj4Leaflet optional

[rgov]

It was a unclear to me from the README that Proj4 and Proj4Leaflet were required dependencies; they are described as "extra" dependencies for an extension to the TileJSON spec.

These files take an additional ~30 KB and about 1/4 of a second more to load. This may not seem like much but if every module requires a bunch of other modules that aren't used, it adds up.