Store certificate globally

[MatteoGauthier]

Hey ! I use local-ssl-proxy a lot for my dev projects. The only drawback is that I'm always creating new .pem certificate files, so I think it would be interesting to have a global configuration.

For example, to avoid adding the two flags --key and --cert, local-ssl-proxy could check the ~/.local-ssl-proxy/ folder to find a certificate. This would make things simpler

That's just an idea, tell me what you think. Have a nice day 😃