Added render utility

Changed the `avecado` command so that it can either make vector tiles or render raster tiles.
Yup, I agree... the difficulty is in figuring out how to do that without uglifying the code and making it difficult to follow by having a load of `try { ... } catch (...) { ... }` blocks around everything. Hopefully, we can leave this for the moment and, if anyone finds a confusing message, try to make the messages themselves better or stub out some of the external library methods to decorate the error messages.