pass options through to DatArchive; update readme

[garbados]

Addresses issue #13 by passing the dat attribute of the opts parameter of indexArchive and unindexArchive to DatArchive as its second parameter. I've also updated the README to reflect this change.

[garbados]

Even with this PR, you won't be able to pass options directly to Dat until this node-dat-archive PR is also merged. Even then, the interface will be a little clunky:

await webdb.indexArchive('dat://foo.com', {
  dat: {
    datOptions: { ... }
  }
})

[pfrazee]

Appreciate the PR! I agree this is a bit clunky. I was thinking about ways to clean it up and then thought, why not instantiate and pass in the DatArchive instance yourself?