RFC: Add months and years to `Duration` units

Obviously this is tricky to implement (due to the unreasonable complexity of reality!):

- [Improved implementation of calculating diff in months and years by dovadi · Pull Request #281 · bitwalker/timex](https://github.com/bitwalker/timex/pull/281)

... but it would still be nice to do, if possible, feasible, reasonable, and generally desired.

I'm willing to help, at least a little, but I'm pretty new to Elixir, completely unfamiliar with the Timex internals, but somewhat *aware of* the heroic effort, and some of the attendant details, involved in writing and maintaining a good date-time library like Joda-Time, Noda Time, and, AFAICT, this one!