Requiring match at a specific index in Rule based MustMatch tests

[gfs]

With #472 we add the ability to include tests directly in Rule object JSON. These tests consist of an array of strings which must/must not match the rule.

A user may want to further have validation that a match occurs at a specified index or occurs a certain number of times.

I'm opening this issue to solicit user feedback if there is value in implementing an additional field must-match-with-indices or the like that would provide this.