Feature: Allow message popup in Mermaid

[lmangani]

We should experiment with clickable items in Mermaid, ie:

Link

 A(Start)-->B(Do some stuff); 
  B(Take some rest)-->C(do more);
  click B "http://www.github.com" "This is a link"

Function

 A(Start)-->B(Do some stuff); 
  B(Take some rest)-->C(do more);
  click B "javascript:alert('you clicked me!')" "This is a link"