Sortable text fields in search aren't creditable by portlets, and require a bounce

we currently require a list of fields in index.sortable.text.fields to be able to make a "Text" field sortable. This property isn't hookable, so for a marketplace portlet (or any portlet) to add sortable search requires a significant amount of effort for the user. We should, at a minimum, make this property hookable so portlets can add their own fields to the list.