Add configuration option to keep temporary files

Keeping the temporary files used by sreview (and its dependency libraries) to do cutting and encoding is very useful when debugging stuff. Currently, the only way to do that, is to manually change the sreview scripts. Maybe this could be a config option.

The onsite encoders are usually temporary machines, that will be wiped afterwards anyway, so disabling cleanup is not an issue there. That's different on permanent machines like vittoria.