Maintain list of archive formats in one place

Maintain list of archive formats in one place

Add a static start() method to FormatActivator to allow outside
classes such as the Archive subcommand of the jgit program to use it
without a BundleContext.  This way, the list of formats only has to be
maintained in one place.

While at it, build a list of registered formats at start() time, so
stop() doesn't have to repeat the same list of formats.

Suggested-by: Shawn Pearce <spearce@spearce.org>
Change-Id: I55cb3095043568740880cc9e4f7cde05f49c363c