Links to callbacks
    A has a tooltip and a callback 
    B has a link and a tooltip 
    C has a long tooltip 
    
    graph LR;
        A-->B;
        B-->C;
        click A callback "Tooltip"
        click B "http://www.github.com" "This is a link"
        click C callback "Tooltip quite long tooltip. So long that several rows are required... Not just two rows..."
    
    Links to urls
    A second diagram that should have its own callback on A even though the node has the same id.
    
        graph LR;
            A-->B
            B-->C
            click A callback2 "Tooltip3"
    
    Issue #272
    
        graph LR;
        A((start))-->B(step1);
        B-->C[step2];
        C-->D{step3};
        D-->E[end];
        classDef green fill:#9f6,stroke:#333,stroke-width:1px;
        class A,B,C,D,E green;
        click B onNodeClick "tooltip"