new "linked rule" feature for chaining related rules

[a-frantz]

Copying from a comment on issue #116 :

This problem sparked an idea for "linking" rules. I imagine the output of sprocket explain MissingMetas saying You should also look at 'DescriptionMissing' (or something to that effect). This linkage would go in one direction. Presumably a user invoking sprocket explain DescriptionMissing has already written meta sections and shouldn't be linked back to MissingMetas.

The Rule trait could have a new method fn linked_rules() -> Vec<&'static str> where the Vec contains other Rule IDs.