Enhance gladLoadGLES2Loader?
In some implementations of EGL, eglGetProcAddress will load both core and extension functions. But this isn't always the case and the spec says it's just for extension functions. In this situation - I noticed it with ANGLE on Windows - a different function is needed to load core functions.

The consequence is that when using EGL, the GLADloadproc passed into gladLoadGLES2Loader needs to know what is an extension function and what is not. It's actually a lot more simple if GLAD makes this determination, since it already knows what is a core function and what is an extension. 

Just wanted to suggest adding a second signature to gladLoadGLES2Loader that accepts two functions, one for loading core functions and one for extension functions. Then the load_GL_ES_VERSION_\* functions use one parameter, while the load_GL_\* use the other.

I haven't looked at making the changes to the generator yet though. Does the glad2 branch handle this differently?
