Add release notes option to tag

[christopher-dG]

Registrator allows one to add custom release notes, and they get pasted into the registry PR for TagBot to consume. PkgDev.tag should offer the same functionality, probably via an optional keyword argument. If release notes are provided, the PR should contain this text:

blah blah blah...

<!-- BEGIN RELEASE NOTES -->
> Here are the release notes.
> - Note how every line is prefixed with "> "! 
<!-- END RELEASE NOTES -->

blah blah blah...