Add release argument to version option of bump

This pull request adds the 'release' argument to the --version option on the bump command. If the current version is a prerelease version, this argument will strip the prerelease segments of the version string. If the current version is not a prerelease, the command `gem bump --version release` is equivalent to `gem bump --version patch`

For example, if version = 0.1.0.pre, the command `gem bump --version release` will result in version = 0.1.0