Allow embedder to explicitly specify a URL to p5

[toolness]

From #45:

We might also want to make it possible to add a data attribute to the widget's <script> tag that allows the embedder to specify a self-hosted version of p5 to use, too, since we currently retrieve that over CDN. This would allow for offline use-cases.

[toolness]

One way to do this is to overload data-p5-version so that it accepts either a version of the form "x.y.z" or an absolute/relative URL to a specific p5 file. Hmm.