Generate URLs relative to cwd instead of dest

Fixes #13.

The reason i want to do this is that it's fairly easy to move the generated HTML file around so it correctly references the files with absolute paths but you can't with relative.

In our build cycle we generate the HTML file and put it in a folder called public and then we expose that from the root URL (/). The CSS and JS resources are exposed through the /js and /css sub folders but since the html file uses relative paths it can't find them. This solves that.