[Feature Request] Add support for angular units

[seth-pierson]

Great Library!

Angular units are missing from the extensive list of units this library already supports. I plan to submit a PR adding support for that category of units.

I saw in #19 there was an example of how to add units, but that link is unfortunately broken. From my looking around my understanding is that I would call NewUnit() and that registers the new unit with the list of supported units. Likewise with NewRatioConversion() for conversions.

I will use volume_units.go and volume_units_test.go as templates for implementing the new units.

Is that a sound plan?