set build_flags: [-v, -i] if empty for compatibility, small updates to doc

[flibustenet]

I didn't know how to differentiate between empty build_flags:[] and not set at all.
I updated the refresh.yml of the readme with the current init.

[opalfroot]

I think the difficulty with this is that you're trying to proxy configuration options through YML then applying defaults which feels like we're overriding configurable behaviour.

The build flags could be set via composition within the main config and have a validation function to determine if the user specific config is runnable. This would be quick and easy and a better alternative to overriding user configuration.

Alternatively a validation function on the main config might be just as simple

[flibustenet]

I'm not sure to see what you mean, i prefer to let you do it if you can