Flag to disable dht or mdns

[mafintosh]

{ dht: false } should disable the dht and { multicast: false } should disable mdns

[RangerMauve]

Would a PR for this be welcome? Maybe using mdns instead of multicast, though?

[gmaclennan]

@mafintosh any plans for this option? I'd like to upgrade Mapeo to use hyperswarm, but for privacy and bandwidth considerations we would like to default to local-network discovery only (e.g. disable the dht).

Use-case: A user is on a phone with limited bandwidth (or expensive bandwidth) and wants to sync over a local network, but does not want to accidentally connect over the internet to another device that might be online at the same time.