[Shock] - Added Jinja filter called 'pathRelativeToPage' to build URLs on top of page URL (Relates to #164)

This PR doesn't actually resolve #164. When I worked on the problem, I found that the Page url property was giving us just the path, e.g., /posts/my-post/. We don't have any official access to the root domain, so there's no way to solve #164 without first deciding where the domain name should officially live. I didn't want to tackle that right at this moment.

With this approach, assuming a website defines it's own data property called homepage_url, a URL can be built as follows:

{{ homepage_url }}{{ "images/photo.png" | pathRelativeToPage }}

We should still make this much easier in the future, but this is enough to unblock some other work.