Merged / Stack Config

[dsully]

Is your feature request related to a problem? Please describe.
I have a need (in a corporate environment) for a global list of allow + deny list, but also the ability to add to the exclusion list on a per-repo basis.

Describe the solution you'd like
Two different approaches I can think of:

1. Allow passing --config multiple times and merge the configuration (order might matter here?)

2. Allow for passing --config but also look for <cwd>/deny.toml (and the other locations in pub fn get_config_path and do the config merge with the local key/values having precedence.

Changes to ValidConfig.load() would need to be made in either case.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
I can put together a PR if this is functionality that would be accepted.

Thanks!