restart_source configuration option is undocumented

The configuration option `global.restart_source` is undocumented in
stb-tester (it's only mentioned in the release notes) even though this
is an important feature for Hauppauge HD PVR users.

Make `restart_source` property available as `--restart-source` command
line argument, just like other available configuration options, and
mention it in the stbt man page.