Add a UUID convertor

[JayH5]

This is a very common type of path parameter. Django implements this and I've copied their regex for it: https://github.com/django/django/blob/3.0.5/django/urls/converters.py#L25-L32

This should maybe be merged after #902.

[erewok]

I support this idea. We have one of these in our projects: it's a pretty common requirement for us.

[tomchristie]

All very neatly done, thanks so much!

Needs a docs update too, at https://github.com/encode/starlette/blob/master/docs/routing.md#path-parameters but otherwise good to go. 😃

[JayH5]

Thanks @tomchristie. I believe I did update the docs where you are pointing to?

Also, I have now checked, and Werkzeug does implement a UUID converter since version 0.10.

[tomchristie]

Oh, of course you did. That’s so much, lovely little PR!