Add a create WACZ command after that fact.

[ikreymer]

If a crawler crashes unexpected, or user forgot to run with --generateWACZ, there should still be a way to generate a WACZ file for a given crawl.
Should probably add a 'generate-wacz --collection X` command that will start the image just to create the WACZ, and not do any crawling.
Should also support signing and signing params, and generally be equivalent to running the crawl with --generateWACZ