Updates to generate command to support stack.yaml as a target with --from-store

Changelog for 0.18.14:

• PR #1032 Remove old generators and add generation from store for output type stack/stack.yaml by @alexellis

Commits
7a39d63 Validate the --output flag in PreRunE by @alexellis
b35e24f Add stack.yaml output for generate --from-store by @alexellis
1d9edf3 Validate the --api flag for generate by @alexellis
b8a3dcc Delete Knative Serving schema by @alexellis
56c1bb1 Remove Knative generate code paths by @alexellis

Changes: 0.18.13...0.18.14

Generated by Derek