release-version.sh: support extra arguments

The maven-release-plugin lets you pass extra arguments to child
executions using the '-Darguments=' flag.

In my case, I need it to pass '-Dgpg.skip=true' so that we can cut
custom releases of third-party software (specifically nar-maven-plugin)
without signing the release.

To support this, add the --extra-args and --skip-gpg options.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
release-version.sh: continue in case of no changes

Particularly when running outside batch mode, we might reset to the same
SNAPHOT version afterwards, so there may actually be no changes to
master at all. In that case, don't try to commit, because otherwise the
'git commit' command will return a non-zero error code, terminating the
script early.