Add helmfile test sub-command (#150)

**Feature**
An additional sub-command to the helmfile binary; helmfile test

**Why**
Helm provides helm test (https://github.com/kubernetes/helm/blob/master/docs/chart_tests.md) as a method to run automated tests post chart install to ensure things are working as they should be.

It would be nice to be able to run something like helmfile test against a particular helmfile in order to run helm tests against all charts/releases defined in the file. Either as part of the sync (i.e. helmfile sync --test) to be ran immediately after the corresponding chart is installed, or as a separate command ran after a sync (i.e. helmfile test).

A chart without tests will exit with a 0 status, so it can be safely ran against any charts.

**Notes**

`--cleanup` (bool) & `--timeout` (default: 300) are available as first class arguments. Additional arguments can be passed to the helm binary as with other sub commands using `--args=`

Resolves #144

helmexec/helmexec.go

@@ -11,6 +11,7 @@ type Interface interface {
 	DiffRelease(name, chart string, flags ...string) error
 	ReleaseStatus(name string) error
 	DeleteRelease(name string, flags ...string) error
+	TestRelease(name string, flags ...string) error
 
 	DecryptSecret(name string) (string, error)
 }