feat(url-viewer): add url-viewer example

This commit restores the url-viewer example that has been previously
removed from bpmn-io/bpmn-js.
fix(simple-commonjs): upgrade grunt-browserify
feat(modeler): add modeler example

This commit restores the modeler example that has previously been
removed from bpmn-io/bpmn-js.
docs(README): link new examples