Add additional vertex attributes

[McManning]

Add more per-vertex data support - assuming it doesn't massively impact performance further (see #5)

• Per-vertex color
• UV coords (Maya supports up to 8, necessary here?)
• Binormal/tangents?

[McManning]

For performance reasons, I think I'll limit the "safe" version to one UV channel. For the faster C structs version (which maybe should be an envvar or global settings toggle for now?), I'll try to support as many as possible and possibly split normals - depending on whether I can grab that CustomData entry correctly or not.