Allow for a "subsection" element in the dictionary

[nusbaume]

It has come up recently about the possible benefit of having some sort of "subsection" element in the dictionary that would allow for the additional grouping of standard names beyond the broader section element that we have currently.

Adding this additional tag would likely require updating the current XML schema as well as the various python-based tools in this repo, but should be doable.

[mkavulich]

I don't see any downside to implementing this. This would be necessary for our preferred solution to #46. My only question is, would this be a single level of subsection, or would it allow for arbitrary nesting?