On GitHub, I found a project called Regal for OpenGL.
Can this be used to write OpenGL code for all platforms? Are there tutorials explaining how to use this framework?
Regal OpenGL can be used to run any version of OpenGL (1.x, 2.x, 3.x) on any platform.
You need not change your current OpenGL code. Instead of linking with the OpenGL lib you have, link it against Regal OpenGL. Use #include <GL/Regal.h>. Link with regal32.dll (Windows), libRegal.so (Linux) or libRegal.dylib (MacOS X).
It isn't stable as yet. Give it time before you use it for commercial projects.