Allow scripts in browser preview.

This allows a single script or array of scripts, each as a filepath or url, to be loaded into the HTML.

If the script is a filepath, it will be loaded inline via `load_utf8()`, otherwise it will become the `src` attribute of the resultant script tag. This helps avoid cross-origin behaviours and requires less setup to get scripts working.

My python's not the best, but I tried to be consistent with the style.