packer, etc: added Help method to Command interface

This will be used for long-form help when "packer help foo"
or "packer foo --help" is called.

command/build/command.go

@@ -10,6 +10,10 @@ import (
 
 type Command byte
 
+func (Command) Help() string {
+	return "help"
+}
+
 func (Command) Run(env packer.Environment, args []string) int {
 	if len(args) != 1 {
 		env.Ui().Error("A single template argument is required.")

command/build/command_test.go

@@ -22,6 +22,13 @@ func testEnvironment() packer.Environment {
 	return env
 }
 
+func TestCommand_Implements(t *testing.T) {
+	assert := asserts.NewTestingAsserts(t, true)
+
+	var actual packer.Command
+	assert.Implementor(new(Command), &actual, "should be a Command")
+}
+
 func TestCommand_Run_NoArgs(t *testing.T) {
 	assert := asserts.NewTestingAsserts(t, true)
 	command := new(Command)