feature: Rekey one or more peers in the config

[xaner4]

There should be a way to regenerate keys for one or more peers in the config in case if stolen keys.
This should be done with a easy to use subcommand like so

For one peer

$ wggen rekey peer --name <peer> --endpoint eample.com 

For server.

$ wggen rekey server --endpoint eample.com 

Does it make sens to also regenerate all keys for all when server changes? Especially since you anyway need to change the config on all hosts?