Ability to change listening address

[Aster-the-Med-Stu]

I am deploying simple-vpn in a docker environment, and find out that the listening address is hard-coded in source code and not able to be changed :(

I know I could forward port but still hope this could be implemented in simple-vpn itself.

[skx]

If you were to submit a pull-request to allow this to be configured I would merge.

Otherwise I think this repository is mostly "done" and should be archived.