Fixes #176, #3
@johnagan, @chrisblossom, I can think of three ways (there could be more) not preserving some of the recent builds might backfire:

• someone's using a website during deployment and requests split chunks previously not loaded that are no longer there
• if a dev decides to cache index.html with a service like Cloudflare, and a split chunk was not requested before and therefore not cached, and is now requested.
• if there was not enough time to load everything, service worker would try to load the rest during the next visit?

It's still a draft because I don't know if you're interested and if I'm moving in the right direction. So far I save the list of assets from previous builds in a file, filename and path are configurable. preserve option is only handled on initial builds, should it work during watch rebuilds as well? Potential improvements:

• allow preserve to be a number (maxAge) - could be confusing if in the future we decide to add nPreviousBuilds additional option alongside maxAge
• remove duplicates from assetList with a Set - unless it does not improve performance of del