support spaces in path names

[sstarcher]

Add support for spaces in paths. exec output should be escaped or quoted.

exec: helm secrets dec /path/has space/helm/secrets.yaml
/path/has space/helm/secrets.yaml doesn't have secrets.yaml file.
exec: helm diff upgrade --allow-unreleased chart stable/chart --values /path/has space/helm/values.yaml --values /path/has space/helm/secrets.yaml.dec```

[AnatolyRugalev]

Could not reproduce this issue at current version of helmfile. Looks like this bug was automatically fixed with introduction of values.yaml rendering functionality (#216). Could be closed IMO