Enable use of a different proxy for .onion addresses.

This implements --onion (and --onionuser/--onionpass) that enable a
different proxy to be used to connect to .onion addresses. If no main
proxy is supplied then no proxy will be used for non-onion addresses.

Additionally we add --noonion that blocks connection attempts to .onion
addresses entirely (and avoids using tor for proxy dns lookups).

the --tor option has been supersceded and thus removed.

Closes #47