Formalize config.libs= as a public API

Making `libs=` a public API allows for safely setting additional load paths in application spec configuration (e.g. `spec_helper.rb`). By allowing the setting of `libs` in an application-level spec configuration file, this allows us to move this particular piece of application configuration out of the `.rspec` file. That way, we can reserve `.rspec` for user- or environment-specific configuration.

A specific use case for this would be to make it easier to keep `.rspec` out of version control while still being able to add additional load paths to the test suite configuration.