text: Title: Add or extend template to allow more control over window size
  Body: At present, the ``plotly_app`` tag inserts an iframe that extends to 100% of the width of its parent, and sets its height as some (user controlled) relative fraction of the width.
  
  This needs to be extended (either with more arguments for this tag, or with a different template tag) to allow more user control over the size.
  