Allow CORS headers to be sent

[rose-m]

First of all thanks for the great tool being a really nice replacement for http.server on steroids 👍

One thing I'd love to have is the support for CORS headers / enabling CORS for the server.

Background:
In some places I'd like to maybe serve a small script or similar to a web page (e.g. to inject some arbitrary code). However, browsers will block loading the resource if the server does not send appropriate CORS headers.

Potential solution:
Since updog uses Flask it might be as easy as to bring in https://github.com/corydolphin/flask-cors and add an additional command line argument, e.g. --cors to enable this behavior (as an opt-in).

If you in general agree on this being a good idea I could come up with a PR.

Downside:
It would increase the list of dependencies of updog to include flask-cors and make the installation bigger.