Add support for a "manual" SCM Adapter

[DamnedElric]

The main use case is being able to release using unsupported SCMs. But this is also a useful workaround for complex git setups: the plugin uses "exec" to execute git, which ignores shell aliases, which breaks some git setups.

When using the following configuration, gradle-release will ask a few extra questions during the release process, and will give the user the chance to commit and tag things by hand.

release {
    scmAdapters = [
        net.researchgate.release.ManualAdapter
    ]
}