[html-formfu] new non_param() method

[Carl Franks]

I've added a new non_param() method to all fields.

Setting it to a true value causes that field's values to be ignored by
param(), params() and valid()

Sometimes I've found myself doing:
    my $param = $form->params;
    delete $param{submit};
    # do something with $param ...

Now, setting non_param(1) on the Submit button will make the 'delete'
unnecessary.

The value of non_param() has no affect on the behaviour of indicator()
or of any constraints or other processors - they'll all still work as
expected.

See the test file t/elements/field_non_param.t for an example.

Cheers,
Carl