Allow passing SSL Cert/Key when using HTTPS

[vonWolfehaus]

What problem does this feature solve?

I actually need the CLI arguments to overwrite the key and cert of the devServer config that this PR addressed, because I have different SSL requirements depending on the environment. Specifically, no SSL for test but use my own certs for development which has unique requirements.

This is essentially re-opening #1226.

What does the proposed API look like?

The addition of --key, --cert and potentially --cacert to vue-cli-service serve.

These values would be optional when --https is used, but would infer --https from the presence of a cert/key.