Add GeoJSON as a Provider

[fawick]

I wonder whether it would be of interest for tegola to have a mvt.Provider for GeoJSON files.

(Again, this is related to #160 and #161, so I apologize for spamming three issues for these at the same tiem, but since these are three very different formats I'd rather keep the pro-and-contra discussions and evaluations for use in Tegola separated.)

[jj0hns0n]

@jyutzler is beginning to work toward this.

[jyutzler]

Sort of. I have produced a library that demonstrates a common model that supports both WKB and GeoJSON. If this library is approved, it could be used to add GeoJSON support to Tegola.