Add command-line options to support TLS.

I needed to use LiveReload with TLS, so I added options to pass a path to a certificate and key so `tiny-lr` will serve over HTTPS.

The diff is a bit messy; the two commits clarify things a bit. The first pulls the lines to start the server into a function `startServer`. That way, the second commit can call that add TLS support directly, calling the `startServer` function after it reads the certificate and key files.