Apply similar patch to a similar heading regex.
Follow-on to f052a2c.

Test: Add a test case to demonstrate the slower blow-up.

Problem:
replace(/X+$/, '') is vulnerable to REDOS

Solution:
Replace all instances I could find with a custom rtrim

Help wanted.

Spec:
No leading whitespace within a paragraph

Fix:
I strip leading whitespace on each line while rendering paragraphs

Unanticipated problem:
Causes the (previously failing but hidden) CommonMark 318 to fail.
I added that to the list of expected-to-fail tests.

This commit addresses a failing header test case noted in 943d995
whose root cause was paragraph rendering not up to spec.

Sketch implementing text regex as a linear-time RegExp imitator.
- A few nits here and there
- I haven't tested all of the offsetOfX routines, so 'npm run test' hangs on some bug