[Feature] IPv6 support

[pkittenis]

Is your feature request related to a problem? Please describe.
It is not possible to use IPv6 addresses, use host names that only resolve to IPv6 addresses, or prefer IPv6 addresses for host names that have both v4 and v6.

Describe the solution you'd like
Allow IPv6 addresses to be used by the library. Have a flag to enable/disable IPv6 for host names that resolve to both.

Describe alternatives you've considered
Library only supports IPv4.

Additional context
N/A

[sasa-tomic]

I would love if parallel-ssh had support for IPv6 because we wanted to use it on a network that has only IPv6, and it did not work :(
So I'll have to come up with something else to use instead. But parallel-ssh seems to be clearly superior than the alternatives.

[pkittenis]

This is the current priority for a new feature.