Add Let’s Encrypt!

[tisba]

This PR adds Let’s Encrypt to automatically obtain a TLS certificate for your FRITZ!Box!

To use, simply provide --auto-cert --domain fritz.example.com. See README for more details.

In other news:

• add --insecure to ignore broken/expired TLS certificates when talking to FRITZ!Box
• add --bundle as an alternative for --fullchain and --key
• add a simple verification of the installed TLS certificate. In case you are not running your FRITZ!Box TLS on port 443, use the --tls-port option.