Auto-register fonts in MAPNIK_FONT_PATH

`MAPNIK_FONT_PATH` can be set in the environment to instruct Mapnik to load fonts from a non-system, non-stylesheet-specified location.

/cc @springmeyer
I see you are not passing the optional second arg of `{recurse: true}`. Is that intentional? I think non-recursing is good but just wanted to check if you'd considered this.