Enable developers to implement their own PropertyValueConverter

Hi,

I think that the ability to implement my own Strongly-typed model property value converter  is a nice option.

I decided to try and implement this on my own. I added an new prevalue property called #OverrideDefaultPropertyValueConverter#. If checked, than the #ArchetypeValueConverter# will not apply, so one can implement his own property value converter.

I thought you might consider adding this to the Umbraco package