Support customising the release notes generator

**Is your feature request related to a problem? Please describe.**

I'm trying to compare the semantic-release tool and Auto. The changelogs/release notes generated by both are slightly different.

Semantic-release by default adds all commits to the release notes, whereas Auto by default adds only 1 sample commit per PR and PR link to the changelogs. I think it'd be great if there could be a way to customise generation of release notes.

**Describe the solution you'd like**

One simple solution is to add a configuration support .autorc file for the type of release note and support it natively. Another way (maybe better) could be to add support to `'@semantic-release/release-notes-generator'` as a plugin.