Feature: Allow to pass storage object to Umzug constructor

[jukkah]

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

const umzug = new Umzug({ storage: new CustomStorage(...) })

Implements #77

[mention-bot]

@jukkah, thanks for your PR! By analyzing the history of the files in this pull request, we identified @sdepold, @alubbe and @thanpolas to be potential reviewers.