Clarify treatment of backslashes and square brackets with respect to embedded links

[ghost]

Section 3.9.5, “Messages with embedded links”, describes a custom syntax for embedded links, and it says that square brackets that are not part of embedded links should be escaped. But it’s not clear whether a backslashed "]" should be considered part of the link text, interpreted as a literal "]".

This is not only insufficiently precise; it also doesn't match GitHub Flavored Markdown's treatment of brackets and backslashes in links. The behavior we want is for this input:

to produce this output:

Unfortunately the spec can't just say "Match GFM's behavior for hyperlinks" because GFM's link syntax includes features we don't want, like hover text. So I'll have to write words to explain exactly what we want.

[michaelcfanning]

Suggest document a regular expression that handles identifyinkg links. Chris M. is developing one.

[ghost]

@michaelcfanning The rest of the spec uses ISO14977 Extended BNF syntax, so I'll use that here too. In fact, the spec already has EBNF for the link syntax, but the proposal I just sent out in mail is slightly different, so I'll need to tweak it a bit.