Formalize config.libs= as a public API

Created by: jtrim

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.