Let theme authors decide the version of the data supported

[humitos]

Add a <meta name="readthedocs" content="version: 1.0" />

This will let the client how to expose the window.readthedocs object. The client may need to do an extra call to the API to get the data in the structure the theme is expecting.

See #60 which is the one that will use this to keep compatibility.

[humitos]

If there is not meta tag in the HTML, we don't expose the window.readthedocs object and we don't trigger any of the custom events.

This way, we force the user of our structure to clearly communicate the version they support.