Feature request: Flag for custom IP for "dsnet add"

Sure, one can edit /etc/dsnetconfig.json, set a custom IP address and run "dsnet sync". But it would be nice to have the ability to config a custom IP address when adding new peers. Because auto-assigning the lowest available IP address is not always wanted. 

E.g. something like  `dsnet add myhost --ipv4 10.80.54.100` would be nice.


