Allow to pass storage object to Umzug constructor

[koistya]

class Storage {
  constructor() { ... }
  logMigration() { ... }
  unlogMigration() { ... }
  executed() { ... }
}

const umzug = new Umzug({ storage: Storage });

[jukkah]

Hmm... Why not? 👍 Pull request is more than welcome!

[randomstuff]

Is it useful to pass the storage class instead of the storage instance? I'd want to do that instead:

const umzug = new Umzug({ storage: new Storage() });