post-processing after querying geometry?

[haoliangyu]

Thank you for making this awesome project. I have been using a fork in my mapping projects and it works very well.

One reason of creating a fork is that sometime I would like to modify the geometry for tiling but the PostGIS doesn't provide enough flexibility. So in this commit, I add an optional post-processing function to the render() and every geometry returned from the database could be further decorated there.

If you think this is beneficial to all users and good to merge into here, I can submit a PR for it.