Make purge optional when running "helmfile delete"

`helmfile delete` has been implying `--purge` but it is not the case since this change.

The new behavior is `helmfile delete --purge` to actually purge releases.
Run just `helmfile delete` to delete releases but not purge them.

Resolves #71

helmexec/helmexec.go

@@ -10,7 +10,7 @@ type Interface interface {
 	SyncRelease(name, chart string, flags ...string) error
 	DiffRelease(name, chart string, flags ...string) error
 	ReleaseStatus(name string) error
-	DeleteRelease(name string) error
+	DeleteRelease(name string, flags ...string) error
 
 	DecryptSecret(name string) (string, error)
 }