* Add analytics munger w/ munge heading
* More link autofixes
* Allow running a subset of munges
* Fix repo root detection
* Only process non-preformatted blocks
* Gendocs no longer adds the analytics link; mungedocs does that in a
second pass.
All munges now start with `<!-- BEGIN MUNGE:` and end with `<!-- END MUNGE:`.
This lets me (in a followup) filter them better to normalize contents during
verification of generated docs.
