Add option to serve an https endpoint instead of http

[halberom]

Describe the feature:
Some companies/solutions require services to all use TLS/HTTPS. At present goss only serves HTTP.

Describe the solution you'd like
Add ability to serve an https endpoint given a certificate.

E.g.

goss --serve --protocol https --cert-key /path/to/cert.key --cert /path/to/cert.crt

Describe alternatives you've considered
Use a reverse-proxy like caddy to sit in front of goss.

[aelsabbahy]

Makes sense, marking this as approved. I haven't looked too close to Go's net/http to see how well it's supported in the stdlib. I would be against pulling in a third party https service into the package though.