add support for stroking atoms

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The design document for balancing-chemical-equations makes this request for the rendering of molecules:

For all atoms, please render with stroke=’black’, lineWidth=1.

There is current no support for stroking atoms in nitroglycerin. We'll need to add this to the various molecule types in js/nodes/ (e.g., C2H2Node). Add an options parameter to the constructor, and pass options to each AtomNode.

Client code would then look like:

var molecule = new C2H2Node( { stroke: 'black', lineWidth: 1 } );

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Support added, closing.