Add a lint_strictly setting

[ErinCall]

I'm a big believer in treating warnings as errors. Sure, many warnings are false positives, but if a normal build has warnings, you're likely to overlook any new ones that indicate a real problem.

For the sake of the -Werr aficionados out there, add a lint_strictly setting¹ that sends the --strict flag to helm lint.

¹ Just calling it strict might be ok, but the struct field in internal/helm.Config should definitely have "Lint" in the name somewhere since the setting is specific to that command.