Add render.js endpoint

This PR adds a new endpoint: render.js. The intention of the endpoint is to be able to execute JS code within the context of the page and returns the results. This allow us to support some use cases like:

 1. Extracting subsections of the HTML code.
 2. Use JS to calculate coordinates of elements in the page.
 
This is an initial version and it is fairly simple, some additional features we can add later are:

 * Allow the user to define JS libs to include (globally or per request), i.e we can add jquery.
 * Integrate render.js with render.json in a similar way to the other endpoints.
 * Add support to render.png to only render a subsection of the page.

I added some basic tests and documentation to the PR.