Commented Feature: Allow for custom IValueFormatters [12364]

I have some cases where I override ToString on my class under test but I'd prefer to use the DefaultValueFormatter as it display nicer formatted results for me (though there are a few properties I would like to omit). There a number ways to fix this but in the end I think allowing the user to setup their own IValueFormatter(s) would be great.
Comments: Associated with changeset 81016: The AttributeBasedFormatter now also works in Silverlight and WP7. WinRT doesn't support enumerating assemblies, so we can't support this feature there.