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Add support for path arguments and sub-resources

This commit adds support for path arguments to be specified in a
resource path:

    class UserProfile < Acfs::SingletonResource
      service UserService, path: 'users/:user_id/profile'
    end

Path arguments also work on non-singleton resources. They will be
taken from either given parameters to query methods like `find` or
from resource attributes when available. Otherwise errors will be
raised.

This commit also lays some foundation for custom per-action different
routes and features depending on path modification.