Support specifying a summary message for each version

[edmorley]

Hi!

We currently use auto-changelog (our changelog) and it works great for displaying the commits that are present in each release.

However sometimes the list of commits isn't entirely sufficient on its own, since:

• it's often very long, and is ordered in commit date rather than in order of relevance to the end user
• the way commit messages should be worded doesn't necessarily match the way one would choose to explain a new feature to an end user (and doesn't give the chance to highlight examples/link to docs etc)

As such, we'd like to be able to optionally include a release summary for certain releases (mostly new major versions), that can give a couple of paragraph explanation about the new release (explaining exciting new changes or linking to the migration guide etc), prior to the usual list of commit messages. We'd like to be able to include this in the changelog (rather than say as a separate docs/blog page) since the changelog is what is used by the bots that automatically open PRs to update people's dependencies, and is likely where manual visitors to the repo would also look first.

One way to implement this might be to use the commit message body of the commit that bumped the version.

For example this commit:

v9.0.0

Here is a summary

...would result in:

### Changelog
All notable changes to this project will be documented in this file. Dates are displayed in UTC.

Generated by [`auto-changelog`](https://github.com/CookPete/auto-changelog).

#### [v9.0.0](https://github.com/neutrinojs/neutrino/compare/v8.3.0...v9.0.0)
> 20 October 2018

Here is a summary

- Commit message foo
- Commit message bar

Does this feature sound reasonable / the best way of implementing a summary? :-)

[edmorley]

Thank you for adding this! :-)