Revisit artifact rule path patterns

[lukpueh]

Description of issue::
Rethink the behavior of artifact rule path pattern filtering, especially if no artifacts are filtered by the pattern.

Current behavior:
All artifact rules take a pattern argument that is used to filter artifacts reported by a link.
If the pattern does not filter any artifacts the rules as practically not applied.

This issue was already pointed out in the discussion in in-toto/in-toto#43 and is also described in the docstrings of the artifact rules verification functions, e.g. for the MATCH rule:
https://github.com/in-toto/in-toto/blob/0beaf5b131b5860e8bf0bb059c9f97a0736851b2/in_toto/verifylib.py#L451-L461

Expected behavior:
Expected behavior is open for discussion. My suggestions:

• each pattern must match at least one artifact
• extend the rule syntax to indicate if the pattern can/must match ?, +, * artifact(s) (c.f. glob characters)