PropertyVariableInterpolator add support for IConverterLocator (patch included)

Add support for IConverterLocator to PropertyVariableInterpolator (see patch) to work with known IConverter for properties.

Sample code:
getString("created", Model.of(item));

properties:
created=Item created at ${createDate} by ${user}.

Currently createDate (and any other property) is always converted by toString() instead of respecting IConverter to get properties localized.