feat: groups in toc

[rhendric]

Fixes #582

See docs/CONFIG.md for usage, but this is pretty much just as straightforward an implementation of #582 as I could manage.

[coveralls]

Coverage decreased (-0.3%) to 77.959% when pulling c2ab10d on rhendric:feat/toc-groups into b486988 on documentationjs:master.

[rhendric]

For the record, something—I don't know what—is wrong with the coverage report. If I remove one of the lines that this report claims aren't being hit, my new test fails. Is it possible I wrote the test incorrectly such that its contribution to coverage isn't being collected?

[tmcw]

Coveralls just fails whenever coverage decreases, it's too picky in that sense. I'll reconfigure it so that it doesn't flag all the PRs in this repo.