support a whitelist of owners for validate

[jjmschofield]

To help people support the schema at https://docs.github.com/en/github/creating-cloning-and-archiving-repositories/about-code-owners

Where in:

"Invalid syntax includes inline comments and user or team names that do not exist on GitHub."

In a config (somehwere, somehow) create a list of allowed owners and make the validate command respect it.

{
 ...
 "validate":{
 ...
 "owners": {
 "allowed": [
 "@bob"
 ]
 }
 }
}