function getNodeName(markup) {
  return markup.substring(1, markup.indexOf(' '));
}
Extracts the `nodeName` from a string of markup.

NOTE: Extracting the `nodeName` does not require a regular expression match
because we make assumptions about React-generated markup (i.e. there are no
spaces surrounding the opening tag and there is at least one attribute).

@param {string} markup String of markup.
@return {string} Node name of the supplied markup.
@see http://jsperf.com/extract-nodename