Use more flexible, hierarchy-friendly format for skill extractor list input

[thcrock]

Our current skill list format is very ONET-centric. Other skill lists can be expressed in this format, but highly hierarchical ones (like ESCO) cannot while retaining their structure. We should make this format more flexible so it's easier to plug in lists like ESCO.

We should still be default only use the leaves as strings to search for in job postings, but once they are tagged we can do more analysis of the tagged skills in the hierarchy later.