Added per-vertex color support to OBJLoader

Some programs export .OBJ files with per-vertex color values rather than using materials. The values are tacked onto the end of a vertex line. E.g.

Without color data:
v 1.463737 -1.050420 -0.000000

With color data:
v -0.104546 0.865881 -0.440595 0.498039 0.498039 0.498039

This commit adds support for parsing the latter format.