Review our CommonMark deviations

[revin]

GitHub seems to have quietly updated a lot of their parsing behavior. While looking into #390 I discovered that they no longer support spaces in image/link URLs or the so-called (by us) "relaxed" link reference specifications (where the link reference URL was allowed to appear without a blank line between it and the preceding paragraph).

There might be more, so we should review our plugins one by one to see if we still need them.

[revin]

Check this out: https://github.com/revin/markdown-testing/tree/da61a08366f08efd6b2122327976be8068326f3c

I created a markdown document that exercises all of our current special GH quirks handling stuff, and all of it is now CommonMark-compliant 😳

• link & image URLs can no longer contain spaces
• links & images are no longer allowed to be "loose" (i.e., having whitespace between ] and ()
• GH now honors CommonMark's allowance of 1–3 leading spaces on ATX style headings
• link references can no longer interrupt paragraphs
• they no longer translate tabs into series of spaces in fenced code blocks

This is amazing; we can delete a ton of stuff.

[revin]

I'm going to just assume that everyone is in favor of removing these features for marky 12 😛 so if anyone has any objections whatsoever, please say so!

@bcoe @ashleygwilliams ?