A few fixes to inline styles and code blocks

[lilyball]

This contains various fixes, including:

• Inline styles shouldn't work across blank lines.
• Inline styles shouldn't highlight if the close token isn't present.
• [link text][destination] blocks should allow a newline where currently only a single space is allowed.
• Fenced code blocks can start after blockquotes, like other block-level elements.
• Blockquotes shouldn't require the space.
• More flexible fenced code block start/end tokens.

I think this at least partially obsoletes #16.

[MarkLodato]

Friendly ping. Any chance this can be pulled in? There are a lot of good fixes in here. Thanks!

[tpope]

Have you been using them for a while? This is a tremendous amount to audit.

[MarkLodato]

I've been using this for about a year now and consider them critical, particularly "Allow newline after [link text] block" and "Treat fenced code blocks like other block-level elements". The "Don't match inline spans..." and "Better fenced code block matching" are also nice but I only hit them every so often. I never used blockquotes so I haven't tested "Consider lines with..." and "Don't require a space...".

[tpope]

The inline styles stuff is most worrying to me; the current balance is quite precarious. I will try it locally. You can check back in in a couple of weeks or so.

[lilyball]

I've also been using this ever since I submitted it. Admittedly I don't edit markdown in Vim particularly often, but I haven't had any problems.

[MarkLodato]

@tpope Would it help to break up this PR into smaller ones that could be reviewed independently? If so, would you be willing to do that, @lilyball?

[tpope]

Having back-burnered this for 7 years, what I need first is someone to investigate and summarize which fixes are still relevant to master, and which aren't.