Allow rebuilding of an existing tag

[tschulte]

Also see #5.

Using the RebuildVersionStrategy does allow rebuilding an already tagged version. This might be necessary, since building, tagging, pushing the tag, creating the github release, publishing artifacts to maven repos are non atomic operations. At any of the mentioned stages there might be errors due to misconfigurations, network outages, timing, etc. The result is an incomplete release.

Therefore it should be possible to just retrigger the build of a tag. This should

• infer the correct version without re-tagging (solvable by applying the RebuildVersionStrategy)
• check if the github release already exists and only create it, if it does not already exist (don't overwrite potential changes made by the author)
• only publish artefacts not already published (this is not directly part of this plugin, but rather a documentation issue)