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

state/state.go

@@ -356,14 +356,18 @@ func (state *HelmState) ReleaseStatuses(helm helmexec.Interface, workerLimit int
 }
 
 // DeleteReleases wrapper for executing helm delete on the releases
-func (state *HelmState) DeleteReleases(helm helmexec.Interface) []error {
+func (state *HelmState) DeleteReleases(helm helmexec.Interface, purge bool) []error {
 	var wg sync.WaitGroup
 	errs := []error{}
 
 	for _, release := range state.Releases {
 		wg.Add(1)
 		go func(wg *sync.WaitGroup, release ReleaseSpec) {
-			if err := helm.DeleteRelease(release.Name); err != nil {
+			flags := []string{}
+			if purge {
+				flags = append(flags, "--purge")
+			}
+			if err := helm.DeleteRelease(release.Name, flags...); err != nil {
 				errs = append(errs, err)
 			}
 			wg.Done()

state/state_test.go

@@ -546,7 +546,7 @@ func (helm *mockHelmExec) ReleaseStatus(release string) error {
 	helm.releases = append(helm.releases, mockRelease{name: release, flags: []string{}})
 	return nil
 }
-func (helm *mockHelmExec) DeleteRelease(name string) error {
+func (helm *mockHelmExec) DeleteRelease(name string, flags ...string) error {
 	return nil
 }
 func (helm *mockHelmExec) DecryptSecret(name string) (string, error) {