packer: Hook interface, not hooked up yet

packer/hook.go

+package packer
+
+// A Hook is used to hook into an arbitrarily named location in a build,
+// allowing custom behavior to run at certain points along a build.
+//
+// Run is called when the hook is called, with the name of the hook and
+// arbitrary data associated with it. To know what format the data is in,
+// you must reference the documentation for the specific hook you're interested
+// in.
+type Hook interface {
+	Run(string, interface{})
+}