Allow linking version strings in changelog

[weierophinney]

Per #38, the 1.0 spec of Keep-A-Changelog allows linking version numbers using reference-style links:

## [1.0.0] - 2019-06-06

### Added

...

[1.0.0]: https://example.com/releases/1.0.0

Previous versions of this tooling did not support this format, and, in fact, would indicate the file was malformed if it encountered such links.

This patch updates the tooling as follows:

• It no longer errors on encountering linked versions in headers.
• It will now identify versions that are linked.
• It will now omit links at the end of the file when retrieving the oldest changelog entry in the file
• It provides the command keep-a-changelog changelog:edit-links for editing the link definitions at the end of the file.