Add option to generate lib as a single-file

[lautriva]

Call gl3w_gen.py with parameter --stb to merge both header and implementation into gl3w.h

Then just drop gl3w.h and glcorearb.h somewhere in your source tree In one of your C/C++ file, you need to #define GL3W_IMPLEMENTATION before including gl3w.h

More information about the 'single file libs' thing on Sean Barrett's main repository https://github.com/nothings/stb#why-single-file-headers