Unitful.Angle?

Perhaps a callback to #38 I think it would be useful to define an Angle "type".

I've hacked this
`const Angle = Union{typeof(1rad),typeof(1.0rad),typeof(1°),typeof(1.0°)}`
I don't see how this would be inherently different from something like `Unitful.Time`.

This is most useful in terms of dispatch. Structures benefit from concrete types.

Start by reviewing the callback to issue #38 and the existing Unitful.Time definition. Compare the proposed Angle Union with the needs of dispatch and concrete structure fields, then establish what an accepted Angle type should support before implementing it.

Written by the indexing model from the issue text.