YearMonth should have a TypeConverter

[malcolmr]

I'm looking at the changes between 2.3 and 3.0-beta, and one such is #1237, in which we added TypeConverter support for all public types, using ISO patterns or otherwise.

We later added YearMonth, but we didn't go back and add a TypeConverter for YearMonth, even though YearMonthPattern.Iso exists.

Should we? (and should we for 3.0?)

[jskeet]

Good question. I think it's probably simple enough to be worth doing for 3.0.