Minimal support for OpenGL ES

Hi,

first let me thank you for this great utility. I'm now happily using it in [my engine](https://github.com/mosra/magnum), the flexibility of this tool is simply amazing and never seen before.

This just adds the ability to extract macros and functions for OpenGL ES 1.0, 1.1, 2.0, 3.0 and 3.1 and ES extensions. I simply reused the `core`/`compatibility` switch and added two other options `es1` (for ES 1.0/1.1) and `es2` (for ES 2.0+).

I didn't add any template file for these because I'm not able to test them reliably and also each platform has its own quirks so there might not be "one size fits all" anyway. Thus supplying proper template files for GL ES is currently up to the user.

Also, the code doesn't have any check about whether all requested extensions really belong to given GL API, but I don't think that's an issue.