Support Multi-line Actor Descriptions

- Add support for <br/> delimiter in actor descriptions.
- Add actorFontFamily and actorFontSize options to sequence diagram.
- Change default actor description font from times to sans.

Fix #384 #702 #755

dist/index.html

@@ -238,6 +238,9 @@ class A someclass;
 
   <div class="mermaid">
 sequenceDiagram
+participant Alice
+participant Bob
+participant John as John<br/>Second Line
 Alice ->> Bob: Hello Bob, how are you?
 Bob-->>John: How about you John?
 Bob--x Alice: I am good thanks!