allow for initial id in repo create

[BrianHung]

Allows for specified id in repo.create.

One thing: in the conversation about pre-defined ids and about loading from binary, does automerge.from clone the document that is passed in?

I'm wondering if this allows for the following:

const copy = {
  content: new Uint8Array([133, 111, 74, 131, ...]),
  id: ...
}

const doc = am.load(copy.content)
const handle = repo.create(doc, copy.id)

so that the initial value in repo.create shares ancestry with the binary that is passed in.