Support revert commit type

[tschulte]

Revert commits are currently not supported. They will be shown in the change log, but

• When the commit, that gets reverted and the revert where both commited after the last version, the result should be the same as if both commits have never happened (no release, if no other commits, not shown in change log)
• If the revert commit reverts a commit of an already released version, a new version should be created, if the reverted commit was a feature, bugfix or performance improvement.