Provide a way to avoid data-react-* properties

Hi @koistya 

I happened to need *gulp-render* to provide html without the `data-react-*` attributes, so I've added a way to send the content through `React.renderToStaticMarkup` instead of `React.renderToString`.

A user would set `toStatic: true` when configuring the plugin.

I'd be interested to get your thoughts on the appropriateness of this option.