Allow CLI option --config-repos to build off of default list

[mikewilli]

Often it's useful to add another config repo URL to the existing list, but the --config-repos parameter overrides the default repos instead of building on them. This is useful too, but it would be nice to be able to specify if you want this behavior. Perhaps a --use-default-config-repos flag that determines whether to build off of the defaults when using --config-repos. Not sure what the default behavior should be.

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