support for generating changelogs

[luxass]

We are going to replace Changesets with this new package. But before we can 100% replace Changesets we need to support generating changelogs.

I think it could be used in the following way:

release({
  changelog: {
  	enabled: true,
    template: "", // Optional
    // This will allow to filter commits, or do something different.
	generateChangelog: async ({ pkg, commits }) => {
    	return /* Changelog Entry */ null
  	}
  }
})