Add unit type to rotation and skew methods

> Something that allows you to specify the unit for rotation, such as rad or turn `Rematrix.rotate(0.5, 'rad')` perhaps?
>
> — @davidkpiano

### Units

* **deg** — Degrees. There are 360 degrees in a full circle.
* **grad** — Gradians, also known as "gons" or "grades". There are 400 gradians in a full circle.
* **rad** — Radians. There are 2π radians in a full circle.
* **turn** — Turns. There is 1 turn in a full circle.

Source: https://drafts.csswg.org/css-values-3/#angles

Re: #1