add <<create>> support (enhancement)

[jderijke]

Great plugin! It would be nice if object construction would be shown as per UML standard

      _________
---->| :Baby   |
      ---------
           |
           |

https://plantuml.com/sequence-diagram#b2c1d43bde93c643

Generated diagrams are already great, but still reverting to hand drawing when I need to show new objects being created.