Date adapter with immutable D support

[rweng]

Closes #163

Should allow immutable objects like dayjs or iso strings to be used as D. Basically all I did is switch some method signatures to return the new value (old behavior was no return type and assumption that the method parameter is modified).

Also adjusted Date and Momentjs Adapter.

I didn't find any automated tests. Since I reverted a couple of accidentally checked in formatting changes after I built the adjusted datepicker for us, maybe someone should check that everything still works as expected.

[bakgaard]

I came here for exactly this. Can we have it, or has the project gone stale?

[bakgaard]

@rweng - looks good, and I would like to have it too. Did you do something in the meantime to get this running for your apps?

[bakgaard]

@bluee - can you trigger a build for this?

[bakgaard]

Did you merge it in?