Link to diff from last version in changelog

[jkrems]

E.g. the 2.0.1 version header in this repos changelog should be:

### [2.0.1](https://github.com/groupon/nlm/compare/v2.0.0...v2.0.1)

[jkrems]

This could also be an opportunity to move links out of the text, e.g.:

### [2.0.1][]

* Some PR title - **[@jdoe][]** [#42][]
  - [`abcabc12`][] fix: Some commit message

[2.0.1]: https://github.com/groupon/nlm/compare/v2.0.0...v2.0.1
[@jdoe]: https://github.com/jdoe
[#42]: https://github.com/groupon/nlm/pull/42
[`abcabc12`]: https://github.com/groupon/nlm/commit/abcabc123456abcabc123456

This could help make the changelogs more readable outside of real markdown viewers.

Rendered

v2.0.1

• Some PR title - @jdoe #42
  • abcabc12 fix: Some commit message