Add option to skip manifests that depend on missing resources #20

Using the lookup() function added with #3, one might write a manifest that depends on a resource that is only created in the same nyl template call. There should be an option for Nyl to skip generating that manifest as it can not be fully "manifested" until that dependant resource becomes available.

Acceptance criteria:

• If a resource uses a lookup() for a resource that does not exist, there should be an option for Nyl to skip this manifest (and it would produce it once the lookup is satisfied)
• Nyl generates placeholder resources for skipped manifests that can be inspected to determine which dependencies were unsatisfied (might require another CRD)
  • Only a single Placeholder resource should be generated with information about all unresolved resources.
• If the dependency does not exist in the cluster but the lookup() could be fully satisfied in the given manifests, Nyl should resolve the lookup in the locally available data regardless to safe an additional iteration cycle.