CHANGELOG link is broken if the tag and the changelogversion don't match

[richardwhiuk]

If the tag is not the same as the heading in the changelog (e..g if the tag is vX.Y.Z or X.Y.Z-release), but the changelog headings just use the version number, then the link to the changelog from Slack is incorrect.

One option would be to recognize the two different link types:

[<version>]: https://git/repo/tree/<tag>

and

[<version>]: https://git/repo/compare/<old_tag>...<tag>

and use that to extract the tag.

An alternative, rejected, proposal in #34 was to just specify the tag on the command line.