Adding in diff plugin execution

- Simple copy-paste for the most part from sync job.  I had started down
the path of adding in a meta PluginCommand directive and trying to make it
more modular, but in the end there are some small differences between
the execution that were a bit difficult to model and it just got ugly.
Figured keeping it simple would be easier to manage

helmexec/helmexec.go

@@ -7,5 +7,6 @@ type Interface interface {
 	UpdateRepo() error
 
 	SyncChart(name, chart string, flags ...string) error
+	DiffChart(name, chart string, flags ...string) error
 	DeleteChart(name string) error
 }