Version changelog preview

[rmohashi]

Description

Currently, I can only see the result of the version changelog, after running the codelog release <x.y.z> command. The problem here is that I want to see a preview of the generated .md file without deleting all the changes from changelogs/unreleased.

I suggest a method option to the release command, to generate that preview file, like codelog release --preview <x.y.z>.

[emilianoLeite]

Closed by #42