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/exec.go

@@ -79,8 +79,8 @@ func (helm *execer) DiffRelease(name, chart string, flags ...string) error {
 	return err
 }
 
-func (helm *execer) DeleteRelease(name string) error {
-	out, err := helm.exec("delete", "--purge", name)
+func (helm *execer) DeleteRelease(name string, flags ...string) error {
+	out, err := helm.exec(append([]string{"delete", name}, flags...)...)
 	helm.write(out)
 	return err
 }

helmexec/exec_test.go

@@ -149,7 +149,16 @@ func Test_DeleteRelease(t *testing.T) {
 	var buffer bytes.Buffer
 	helm := MockExecer(&buffer, "dev")
 	helm.DeleteRelease("release")
-	expected := "exec: helm delete --purge release --kube-context dev\n"
+	expected := "exec: helm delete release --kube-context dev\n"
+	if buffer.String() != expected {
+		t.Errorf("helmexec.DeleteRelease()\nactual = %v\nexpect = %v", buffer.String(), expected)
+	}
+}
+func Test_DeleteRelease_Flags(t *testing.T) {
+	var buffer bytes.Buffer
+	helm := MockExecer(&buffer, "dev")
+	helm.DeleteRelease("release", "--purge")
+	expected := "exec: helm delete release --purge --kube-context dev\n"
 	if buffer.String() != expected {
 		t.Errorf("helmexec.DeleteRelease()\nactual = %v\nexpect = %v", buffer.String(), expected)
 	}

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)
 }