Support date input components without all three components

[gunndabad]

Currently we always generate day, month and year for a date input; the GDS fixtures have examples for day + month and month + year.

[sussexrick]

@gunndabad FYI our implementation based on yours already has the month and year version, so getting this in the base library would be great.

[gunndabad]

@sussexrick do you support binding to a single model property in your implementation or do you have a property per field?

[sussexrick]

We have a single property and just assume the first of the month. That allows us to switch the day field on and off in our UI with no code changes.