Sequence Diagram Async Arrow Support

[TheSharpieOne]

The async arrows for a sequence diagram, -) and --), as defined in https://mermaid-js.github.io/mermaid/#/sequenceDiagram?id=messages do not seem to be supported

Type	Description
-)	Solid line with an open arrowat the end (async)
--)	Dotted line with a open arrow at the end (async)

Sample:

```mermaid
sequenceDiagram
  Thing1-)Thing1: something to self
  Thing1-)Thing2: something to other
  Thing1--)Thing1: something to self
  Thing1--)Thing2: something to other

Error message (runtime):

sequenceDiagram
  Thing1-)Thing1: something to self
  Thing1-)Thing2: something to other
  Thing1--)Thing1: something to self
  Thing1--)Thing2: something to other

Loading

Error message (img):

how others render it:

Also seen in the first example of https://github.com/mermaid-js/mermaid/blob/develop/docs/sequenceDiagram.md