Change to use Face3 to remove Face4 three.js deprecated usage

Closes https://github.com/maxogden/voxel-mesh/issues/13. This is compatible with both three.js 0.58 (as before) and 0.66 (which removed `Face4`).

(also, unrelated: adds the `mesherExtraData` argument for https://github.com/maxogden/voxel/issues/16)