title: add document with ReactiveMongo with specified _id
body: <p>I'm trying to insert a new document with _id field specified in ReactiveMongo. But all my inserts are executed with Mongo default incremental objectId. Is there a way to specify _id during insert? </p>
  
  <p>Here is my code.</p>
  
  <pre><code>case class MongoId($oid: String)
  
  object MongoId {
    implicit val mongoIdFormat = Json.format[MongoId]
  }
  
  case class Person(_id: MongoId, name: String) 
  
  object Person {
    implicit val PersonFormat = Json.format[Person]
  }
  
  val collection = reactiveMongoApi.database.map(_.collection[JSONCollection]("people"))
  
  def save(person: Person) = {
    collection.flatMap(d =&gt; d.insert(person)).map(wr =&gt; wr.hasErrors match {
      case true =&gt; None
      case false =&gt; Some(person)
    })
  }
  </code></pre>