"localhost" as default for host option can lead to long debugging sessions

[TimDaub]

👋🏼 hey,

this is probably a bit subjective but the fact that the host option defaults to localhost actually cost me a couple of hours debugging right now.

I accidentally set await isPortReachable(22, { ipv4 }) and hence it would never resolve until I spotted the error that I had not set the host option appropriately.
Anyway, I think a better default would be to throw in case no host is set, as it means the function can't proceed.