Fix Setext headings in block quotes

[hamvocke]

This PR attempts to fix #465 (spoiler alert: I've introduced another problem).

I've added a new test that makes sure that this markdown

> Hello
> ----
> this is some other text

is parsed as a block quote containing a h2. I made the test pass but introduced another regression around lazy continuations of block quotes. This is quite a head-scratcher for me so I'm getting this out with the failing test, maybe we can start discussing intermediate results and another pair of eyes helps fixing the remaining regression (or making sense of my overall approach)

[xoofx]

Thanks for the PR! Sorry for not being able to review it quickly, I will try to come back to you later this week-end, pretty busy during this week.

This is indeed tricky, mostly because we currently don't know when we are in a continuation line or not, which makes this detection not easy to do.

[xoofx]

Sorry, just back on this bug, I will try to have a look at it this week-end.
Discovered that it was no longer possible to debug the tests nicely because of a static constructor performing markdown parses, so I have fixed that.

[xoofx]

This should be fixed by commit 8b64ce4