Allow for accessing "Previous Version" in versionFormat

[rocktavious]

Is your feature request related to a problem? Please describe.
I would like to use a custom version format that allows the version header be a link to the changes

versionFormat: '## [{{.Version}}](https://github.com/miniscruff/changie/compare/{{.PreviousVersion}}...{{.Version}})'

So it would emit the markdown

## [v0.2.0](https://github.com/miniscruff/changie/compare/v0.1.0...v0.2.0) 

Describe the solution you'd like
It should be as simple as pipeing the "current version" of the batch command before increment as one of the valid batchData things.

Describe alternatives you've considered
It seems pretty straightforward ask so I had not considered anything things since the syntax for the formatting requires the previous version

[miniscruff]

I commented this on the pull request, but will add it here as well. Because I am rewriting the batch and batch test files I will not be accepting your PR directly but I will add this value during or directly after the rewrite.