Try rendering examples in iframe

Right now each example is a separate React app that renders into the same DOM as the Styleguidist app itself. This approach have some problems:

• We share styles between Styleguidist and user's component, we use some hacks to isolate Stylegudist styles but sometimes styles still affect each other.
• We can't implement a nice responsive design preview, because we can't affect media queries in components without resizing the window.

Possible issues of the iframe approach:

• It could be slower than the current approach.
• Interaction between Styleguidist and iframes could be more complex.
• We'll need to add global styles to every iframe instead of adding them once at the Styleguidst root level.