Allow merge of version update in the develop branch when using fromCommit parameter

[ohubaut]

Currently, when using the fromCommit parameter on the release-start mojo with a hash that is not the head of the develop branch does create the appropriate release branch, but the version upgrade is not merged back into the develop branch, because the new commits are created from a dettached head.

The proposal is to keep the current behaviour as is, to not break backward compatibility, but to add another flaw mergeDevelop (or any better name) that would merge the commit with the release bump to the develop branch.

The typical use case is when we include the use of this plugin in a CI toolchain and that some additional commits are added to develop before the release started.
Most CI allow you start from a previous build, passing the commit hash as a parameter, but the current implementation of release-start leaves the develop branch with the "old" version number is this case.