Add `helm schema lint` command to lint the config file

[applejag]

Running helm schema lint (subcommand name mimics Helm's helm lint) should lint a config file and its input files.

It should parse the input files and reuse the same parsing errors that the helm schema file generation would run.

In addition to this, the .schema.yaml file should also check for unknown fields and log those as warnings.

Finally, a --strict flag is needed to fail on warnings (exit code != 0)