BREAKING CHANGE: allow matching with regex for stacks print_event

[MicaiahReid]

Description

This PR introduces the ability to match on a print_event's values with a regex string rather than just with the previous contains field.

After working on this, I realized there was another way (albeit a kind of clunky way) of doing this without breaking the API. See #380.

Breaking change?

This breaks predicate registration for the if_this condition of stacks print_event predicates.

Previously:

{
  "scope" : "print_event",
  "contract_identifier": "*", 
  "contains": "vault"
}

Now:

{
  "scope" : "print_event",
  "contract_identifier": "*", 
  "matching_rule": { "contains": "vault" }
}

Checklist

• All tests pass
• Tests added in this PR (if applicable)