Wireguard

When configuring a peer, for certain types of peers, it is beneficial to have a QR code to scan during the setup process. This enables that in the core library. However, it is an optional dependency, and as such must be installed via `pip install wireguard[qr]`. Alternately, installing the `qrcode[pil]` library will allow the functionality to be used.

The object structure has been refactored to better represent the WireGuard config structure, which doesn't really have a server vs peer differentiation. Both are peers of each other.

Additionally, a `Config` class has been created to simplify the `Peer` and `Server` classes.

Replaces use of upstream-only (ie: peer -> server) subnet routing to two values, to allow for distinct subnet routing in either direction.