Use authbind on Linux to serve copyparty on port 80 and 443 without root.

Hi,

I just wanted to share that it is possible to serve `copyparty` on port 80 and 443 without root privileges using [authbind](https://en.wikipedia.org/wiki/Authbind) like this:

`authbind --deep python copyparty-sfx.py -p 80,443`

`authbind` will run the program as the current user, but allow `copyparty` to serve on port 80 and 443.

Still, you need to setup some "permissions files" to tell authbind to allow users on your computer to run `authbind` to serve on those ports. This can be simply done for all users on your Linux by running

```
sudo touch /etc/authbind/byport/80
sudo touch /etc/authbind/byport/443
sudo chmod 777 /etc/authbind/byport/80
sudo chmod 777 /etc/authbind/byport/443
```
which will create these two files 80 and 443 and set the permission-bits that tells that all users on your Linux computer can run authbind to serve anything on port 80 and 443. See [man](https://manpages.ubuntu.com/manpages/xenial/man1/authbind.1.html) on how this works.

I think this is one of the easiest way to make a program serve on "privileged ports below 1024" on Linux. Maybe you can add this to the `README.md` to give people yet another option to run `copyparty` on port 80 and 443 on Linux? Or maybe detect that `authbind` is installed and then run it using it for an out of the box experience?

Anyways, thanks for this wonderful program. It is very useful for serving files on LANs.