support font-awesome

[thomasleveil]

Could icon syntax be supported?

https://mermaidjs.github.io/flowchart.html#basic-support-for-fontawesome

graph TD
    B["fa:fa-twitter for peace"]
    B-->C[fa:fa-ban forbidden]
    B-->D(fa:fa-spinner);
    B-->E(A fa:fa-camera-retro perhaps?);