[Feature Request] Support emoji in commit message

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

Some developers like to [prepend emojis to commit messages](https://github.com/arvinxx/gitmoji-commit-workflow) to make them look visual. These commits are similar to regular conventional commits. However, Release Please currently cannot parse such commits.

**Describe the solution you'd like**
I hope release-please can handle that kind of commit messages.

**Describe alternatives you've considered**
It would be great if we could expose the stage of parsing commits to allow custom plugins! (Although this would involve a lot of code changes)

**Additional context**
Some Conventional Changelog configurations:
- [conventional-changelog-gitmoji-config](https://github.com/arvinxx/gitmoji-commit-workflow/tree/master/packages/changelog)
- [@favoloso/conventional-changelog-emoji](https://github.com/favoloso/conventional-changelog-emoji)