have the config know whether a digest path is a combo or not

This will be used in the upcoming recursive combinations feature.
I need to know if a given path is a combo or not to know if I need
to keep recursively processing the digest path or break the recursion
and treat it as a "null source".
Merge pull request #57 from redding/kr-is-combo

have the config know whether a digest path is a combo or not