Add URLSelector

Hi param devs, I'd like to be able to use [paramnb](https://github.com/ioam/paramnb) to create a dropdown of URLs that are limited to one or more domains. Since paramnb relies on param's Parameter subclasses, I figured it would be appropriate to create this issue here.

My complete use-case is for a GET request to be issued whenever a URL is selected from the ipywidgets dropdown in paramnb. A callback might then parse more URLs from the response HTML, and the new URLs could be automatically inserted into the same (or a separate) dropdown widget from there. This functionality would be useful for exploring a large directory tree on a remote server, for example, without needing to download all the files in advance.

I can probably get by in the meantime by using the `callback` kwarg in `paramnb.Widgets`, but this seems like a common-enough use-case to merit its own parameter. Another potential approach would be to extend FileSelector/MultiFileSelector parameters to accept URLs as arguments.