Added options, SVG size, graph fit & scale and return of existing renderer instance

• If a graphviz renderer instance already exists on an element, return the existing instance instead of creating a new one every time.
• Options can be set when the graphviz renderer is created
• Options can be set with the graphviz.options method
• Added options to set SVG width and height and to fit the graph to the resized SVG
• Added option to scale the graph within the SVG