Forgot `.add`

[thomcom]

Hiya! I'm noticing that gl-matrix.mat4 and the docs have the .add function, but here it does not. Was it forgotten? Thanks!

[rreusser]

Good point. It looks to me like they may have just been missed, so I submitted #15 and added sub() as well. There are no tests in place so that I…… haven't actually run the functions… but I mean… it's hard to mess these up…… right? 😬 If it looks good to you, then I can merge and try to get this published.

[thomcom]

Nice, that was fast! Looks good to me.

[rreusser]

It's all set and merged to master, except I don't have npm credentials on me at the moment, so I'll have to publish later.