Markdown links don't always have their urls attached. Named links allow
putting them afterwards. This change allows the reader to understand
this pattern on the tag headers:

    ## [1.0.0]

    This is version 1.0.0

    ## [0.9.1]

    This is version 0.9.1

    ## [0.9.0]
    ...

    [1.0.0]: https://github.com/myorg/myrepo/compare/0.9.1...1.0.0
    [0.9.1]: https://github.com/myorg/myrepo/compare/0.9.1...0.9.0
    ...

Without this change, the generator will get confused by the brackets
around the versions, and complain that "it cannot find the [1.0.0]
tag" on `all_tags` (which don't have the brackets)