Allow configurable defaults as static on Microcosm child

[nhunzaker]

class Repo extends Microcosm {

  static defaults = {
    hydrate: false,
    saveInterval: 5000,
    maxHistory: Infinity
  }

  // Instead of passing defaults here, just use `static defaults`
  setup ({ hydrate=false, saveInterval=5000, maxHistory=Infinity }) {
  }
})