Introduce an "generate-config" command that dumps a default config file

[czechboy0]

Introduce a "generate-config" command that dumps a default config file.

This should emit all the configuration options.

Required ones should have a reasonable value. Optional ones should be commented out, and have their default value there.

With this, the user can more easily uncomment/customize values before digging through docs and the config file syntax.

[simonjbeaumont]

Potentially a generate-config rather than init. I think we can keep init for #448