Update/database migration

[OleLaursen]

I appreciate the automatic table creation thing in db.js, but it's lacking a callback for the things it cannot possibly handle because they are application specific. A simple

if (options.migrate) // or perhaps options.update/options.upgrade
    options.migrate(e);

in the onupgradeneeded callback would be nice. It's a shame to see database versioning actually being built into the spec and then not being able to act on it.

[brettz9]

FYI, I've proposed some alternative structures for schema versioning (based on your schema approach and also on other JSON standards) to treojs/idb-schema/issues/9 . If he has interest to incorporate (of if I am moved to do this in its own repo) and if you are open to it, @aaronpowell, then I could submit a PR here incorporating the independent repo.

Note that I already have an idb-schema branch mostly ready (compared to my latest previous PR or compared to your master), though it is pending changes suggested here and on the proposed changes mentioned above.