Custom attributes on model to facilitate conditional display of entry fields

[pm64]

There are cases where we only display field Foo if the user enters value x into field Bar.

The most common example is probably a dropdown with an "other" selection that causes an additional text input to be displayed.

If #29 were implemented, I could achieve this behavior by manipulating the generated form. But the requirement is so common that specifying the conditional display logic using custom attributes on the model would be ideal.

[Aaltuj]

Please see my response in #29 :)