Minimal support for OpenGL ES.

Currently only XML parsing and function extraction, no ES-specific
changes in template files.
Less optimistic stripping of duplicate commands.

Currently in the gl.xml file there is duplicated glGetPointerv() in GL
4.3 and also glProgramUniformMatrix4fvEXT() in
EXT_separate_shader_objects ES extension. It's thus needed to remove
duplicates also inside the subset itself.
Changing use of version.core to check of version.profile in templates
Merge branch 'duplicate-commands' of https://github.com/mosra/flextgl into gles
Merge branch 'testing' of https://github.com/ginkgo/flextGL into testing
Added glfw3-es template for GLES contexts using GLFW3.

The only changes in comparison to original `glfw3` template are:

 * Removed core/compatibility profile distinction.
 * Replaced 'VERSION_1_0' etc. branch with GLES-specific values.
Merge branch 'testing' of https://github.com/ginkgo/flextGL into gles
Slightly changing syntax of profiles for es version declaration
Update README.md
Merge branch 'testing'
Updating changes in README.md