Decomposing a 3D matrix

First of all, great job with this library - the code is well-documented and clear. I was up late last night looking for a good library for manipulating CSS transforms, and was very happy to find this one!

I was wondering if you would be open to the following (small) features? I can definitely help with some of them, let me know if you are willing to accept PRs on any of these.

- [x] b45c121 — `Rematrix.toString([...])` for converting a matrix array to a CSS string.

- [x] 574e24e — `Rematrix.rotate(...)` that defaults to `Rematrix.rotateZ(...)` (re: CSS spec)

- [ ] ~Something that allows you to specify the unit for rotation, e.g. `Rematrix.rotate(0.5, 'rad')` perhaps?~ (Moved to #2)

- [ ] Decomposition: probably a bigger feature ask, but a way to decompose a matrix and grab specific values such as `translateX`, `skewY`, `rotateZ`, `scaleY`, etc. from it?

- [x] 0817c2d — Typescript definition file

Thanks so much for this library!