text: Title: create a command to create/update mongo indexes
  Body: I'm not sure if MongoEngine supports automatically creating indexes for EmbeddedDocuments in https://github.com/pythonhub/quokka/blob/master/quokka/core/models.py
  
  Example:  Class Comment has a meta class that does not produces any result on the main Content Class
  
  class Comment(db.EmbeddedDocument): 
  (...)
     meta = {
          'indexes': ['-created_at', '-available_at'],
          'ordering': ['-created_at']
      }
  
  class Content(HasCustomValue, Publishable, LongSlugged, Commentable)
  (...)  
      meta = {
          'allow_inheritance': True,
          'indexes': ['-created_at', 'slug'],
          'ordering': ['-created_at']
      }
  
  Creates only two indexes, 'created_at and slug
  