Problem:
Four regexes were vulnerable to catastrophic backtracking.
This leaves markdown servers open to a potential REDOS attack.
Solution:
Refactor the regexes.
For two similar regexes (html) I didn't change the language.
For two similar regexes (noline) I slightly changed the language:
![[[[[[[[[[[]] was accepted by the old noline pattern.
It is now rejected.
All tests pass, though I'm not sure if I've broken something that
was untested.
This addresses #1070 (with #1058 along the way).
Bonus: rename a stray test to use _ instead of -.
