title: Set default date when inserting document with time.Time field
body: <p>In <code>mongoose</code>(<code>node.js</code>) I can define a model schema with a default <code>Date.now</code> like so:</p>
  
  <pre><code>...
  type: Date,
  default: Date.now
  ...
  </code></pre>
  
  <p>How do I achieve the same without having to insert the <code>time.Time</code> every time I create a document with <code>mgo</code>?</p>
  
  <pre><code>type User struct {
      CreatedAt   time.Time `json:"created_at" bson:"created_at"` // Make this field filled automatically with time.Now() every time a document of this `struct` is inserted
  }
  </code></pre>