Pass CleanupOnFail to the Upgrade Step [#65]

I don't love the mismatch between the helm.Config field (CleanupOnFail)
and the setting name (cleanup_failed_upgrade). I do think the setting
name should contain "upgrade" since it's specific to the upgrade command,
but if I make the config field CleanupFailedUpgrade, it becomes the new
longest field name, and gofmt creates a bunch of churn. Is that a good
enough reason...?

internal/helm/config.go

@@ -44,6 +44,7 @@ type Config struct {
 	Release            string   ``                                   // Release argument to use in applicable helm commands
 	Force              bool     ``                                   // Pass --force to applicable helm commands
 	AtomicUpgrade      bool     `split_words:"true"`                 // Pass --atomic to `helm upgrade`
+	CleanupOnFail      bool     `envconfig:"CLEANUP_FAILED_UPGRADE"` // Pass --cleanup-on-fail to `helm upgrade`
 	LintStrictly       bool     `split_words:"true"`                 // Pass --strict to `helm lint`
 
 	Stdout io.Writer `ignored:"true"`