Support --debug flag for helm diff

[cortopy]

When writing my charts I often get a message like:

2020-05-27 20:12:53 CRITICAL: Command returned with exit code: 1. And error message: Error: Failed to render chart: exit status 1: Error: YAML parse error on chart/templates/configmap.yaml: error converting YAML to JSON: yaml: line 15: did not find expected alphabetic or numeric character

Use --debug flag to render out invalid YAML

Error: plugin "diff" exited with error

My problem is that helmsman -debug flag only applies to helmsman commands and doesn't seem to get passed to helm diff

Passing the flag would help enormously when debugging templates