Verify fragments

[ctm]

If a location ends with #fragment, then there needs to be corresponding Markdown code in the location file that will generate the fragment anchor. Adding a consistency check for this would be nice.

[ctm]

In theory, since mdck is already parsing all the markdown files to look for broken links, it should be trivial to also keep track of the anchors that are generated by the various headers encountered. Unfortunately, the algorithm for translating from header text to an anchor is not documented. It is, however, implemented in comrak, although that functionality isn't exposed in a standalone manner, instead it's part of the code that generates HTML.

One possibility I'm pursuing is to have comrak expose that functionality. As such, I'll go ahead and implement fragment verification while that issue is being considered and then finalize my implementation once a decision has been made.

FWIW, I had already looked into (and started playing around) this issue before #2 was requested. I think #2 is actually a more useful feature (especially since it was requested by an actual user), but much of the underlying change to support fragment verification will also allow me to implement #2, so for all practical purposes I'm working on both of them in parallel (although admittedly I'd finish #2 sooner if I hadn't been distracted by a bunch of fragment edge cases).