Make edit-update-site less strict

When we edit an update site, we already have to provide all the
information required to add it should an update site of the specified
name not have been added yet.

Be nice and just auto-add the site.

This addresses https://github.com/imagej/imagej-updater/issues/21
partially.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>