Support optional secure HTTPS mode (opportunistic TLS?)

[clue]

Currently, LeProxy does listen on http://localhost:8080 by default for incoming request to proxy to a destination server. In the HTTP CONNECT proxy mode, it also supports forwarding secure TLS requests. Once #52 is in, one can also access http://localhost:8080 directly in the web server to access its web interface.

Additionally, its web interface should also be accessible via https://localhost:8080 for a secure HTTPS access. This is not to be confused with the above HTTP CONNECT mode, LeProxy already supports accessing HTTPS sites just fine.

Implementation wise this is pretty much done already. However, this is currently using some internal APIs, so it is likely blocked by reactphp/socket#89 before we should integrate this here.