Improve release heading parsing

[benwebber]

If a heading contains a link followed by a literal (text), we attempt to parse it as a release heading.

notabene/src/parser.rs

Line 101 in 44142a7

[Inline::Link(l), Inline::Literal(t)] => {

Any markup or extra text in between elements of the release heading will cause parsing to fail. We can probably emit better diagnostics here than reporting an "invalid section heading".