Add the ability to cache and block the output of custom facts

In the first round of Configurable Facter work, we added the ability to cache and block built-in facts via settings in the config file. There is also demand to be able to cache and block custom Ruby facts. Blocking should be straightforward, but to implement caching, some updates to the methods dealing with the fact cache are needed, since they currently assume resolvers are the source of all the fact data.

In order to keep the feel similar to resolvers, and in response to customer feedback, custom facts should specify a cache group in the form of a sort of tag in the fact, which can be used to cache or block any custom fact with that tag.

In addition, the output of the list-block-groups and list-cache-groups command line options need to be updated to also report custom facts available for caching and blocking.