Fix/set initial static graph

[danielcaldas]

This PR fixes #24, additionally it adds to the Sandbox component an example (with random generated values) on how to properly set positions for nodes x and y props:

decorateGraphNodesWithInitialPositioning = (nodes) => {
        return nodes.map(n => ({
            id: n.id,
            x: Math.floor(Math.random() * 500),
            y: Math.floor(Math.random() * 500),
            symbolType: n.symbolType || 'circle'
        }));
}

Also

• Add .editorconfig
• Config lint rules for test files with eslint-plugin-jest