Use https if 'pfx' option is present

`crypto.createCredentials` (and therefore `https.createServer`) can accept a PFX or PKCS12 certificate via the `pfx` option, so tiny-lr should pass this through if provided.

This is especially helpful on Windows, as keys created via `makecert` can be easily exported to PFX.

(Note: I'll open this same PR against the mklabs repo as well.)