Add Support for 'Extra Identifier' Releases

[josephbwagner]

This feature enables users to produce "extra-identifier" builds, such as RCs for testing. Here's an example of creating SemVer-compliant RCs using the new functionality:

1. Partway through a patch release cycle, there are 32 commits in this SnapshotVersion: 1.7.0-32-b324c0e3-SNAPSHOT
2. Commit change(s) then exec git tag v1.7.0-rc.1 creates this version: 1.7.0-rc.1
   • Note: "Exta identified" / PreRelease builds do not contain commit count or commit hash, like a standard Release build.
3. Commit change(s): 1.7.0-34-7b63a9a9-SNAPSHOT
   • Note: commit count is retained after a RC release.
4. Commit change(s) then exec git tag v1.7.0-rc.2 creates this PreReleaseVersion: 1.7.0-rc.2
5. Commit change(s) then exec git tag v1.7.0 creates this ReleaseVersion: 1.7.0
6. Increment gitSnapshotLowerBound patch level in the repo, then commit change: 1.7.1-1-cc5c1b9e-SNAPSHOT
   • Note: The commit count is reset per SemVer spec for incrementing patches.

[Wosin]

@jeffmay Any chance to get that reviewed? It seems like a nice addition to the plugin.