Add version selector for known URLs

[lucacasonato]

For known URL types (githubusercontent.com, deno.land/x, jspm.io, pika.dev) add a version selector dropdown.

Would be cool if we could share some code with @hayd's udd. Mainly this file: https://github.com/hayd/deno-udd/blob/master/registry.ts.

[hayd]

I was planning on cleaning up that file a little... Is it given a url you want a list of versions?