General integration for extensions

[sgrove]

I'm hacking in drawElementsInstancedANGLE support right now just to make it work, but GD should probably have a general mechanism for

1. Querying for the availability of extensions
2. Retaining extensions, since
   1. In some UA's the extension's functionality stops working if the reference to the returned extension object is GC'd
   2. Functionality is sometimes tied to the returned extension object.

It seems likely that support for each extension will have to be added to GD itself to make things work together. The list of GD-api-supported extensions should be listed briefly in the README with more documentation on the wiki.