Only work on fields with the date validator set

[daniel-ac-martin]

As this module will add in validation when it detects a input-date mixin submission, it probably makes more sense to only work with fields that have the 'date' validator set.

This has the side benefit of limiting the damage of #4.

[daniel-ac-martin]

Fixing this using a special 'input-date' validator.
(Because I believe that template mixins should be modified in a way that will require a special validator anyway.)