Attach Filters to Fields

[rdohms]

As described in #18, validations attached directly to fields are processed against Form (array) data, not the resolved entity, so the filters do not affect the data.

The alternate solution here would be to add the possibility of attaching filters to the fields, so that they would act directly on the form data for non-entity forms. This would be similar to the constraints option.

Some research points to make this viable:

• does altering the form data ensure the same data is pushed along?
• how does this play with entity forms, are the effect seen on the final entity?
• what hook points does symfony provide for this

This is strictly on the Symfony side, so any solutions should be restricted to this bundle, not the underlying library.