Reference links

[zacharysyoung]

This PR correctly formats the Markdown in #13, but creates (uncovers?) an issue with escaped square brackets in the label. I didn't want to change other code, so I've added six round-trip-failure exceptions, just to prove that at least reference links print correctly.

The new issue arises with input like:

[foo][ref\[]

[ref\[]: /uri

being formatted and printed as:

[foo][]

A guard clause at the top of normalizeLabel sees the bracket in ref\[ (from parseLinkLabel) as invalid and returns the empty string, so the link (and title) are mapped to the empty string in the links map; and my code also tries to associate the parsed link with the empty string:

markdown/link.go

Lines 398 to 405 in 868a055

	func normalizeLabel(s string) string {
	if strings.Contains(s, "[") || strings.Contains(s, "]") {
	// Labels cannot have [ ] so avoid the work of translating.
	// This is especially important for pathlogical cases like
	// [[[[[[[[[[a]]]]]]]]]] which would otherwise generate quadratic
	// amounts of garbage.
	return ""
	}

Removing that guard allows those six tests to pass, along with the new ref-link tests, and doesn't affect any other test.

Given that parseLinkLabel allows escaped brackets (I believe it allows all escaped chars), can a label with a bracket be used as a key?

Removing that guard clause also makes the BigTest/nested_brackets test take about 5s on my machine to run. Does that test mean anything, given that there's a 999-char limit on a label?

I'd be happy to reconsider my approach my to reference links.

I also saw a TODO in normalizeLabel about using the normalized label only as a key in the links map, and printing the original label. If that work fits in the scope of this issue/PR, I'd work on that.

Thanks.