Added configurable separator

We use CouchDB a lot for document storage. CouchDB uses dots in some document property names, so we need a way to configure the separator for nested properties. This PR should enable us and others to use `ampersand-view` bindings with CouchDB.

In particular, CouchDB stores attachments with their file names, which normally contain dots:

````javascript
{
  "_id": "some_doc",
  "_attachments": {
    "some_file.jpg": {
      "content_type": "image/jpeg",
      "revpos": 13,
      "digest": "md5-Oa39RRTNvOpPUvhHI6fAwA==",
      "length": 231548,
      "stub": true
    },
    "some_other_file.doc": {
      // and so on...
    }
  }
}
````