Add ability sort testimonials adhocly

[michael-cannon]

Users frequently use company or location meta fields to put 1,2,3… and then sort by them. That's not feasible in the long run for users who use all fields. Also, configuration for such usage can be confusing.

It'd be a benefit to all free plugin users that TW supports adhoc sorting. I tentatively suggest looking into supporting a field like menu_order or the similar that's only populated when the user provides the information. Further, we should add the option to order by menu_order or not by default to general and widget settings.