Implement the debug flag and help command

I'm vacillating about the choice to have separate Config structs in the
`helm` and `run` packages. I can't tell whether it's "good separation of
concerns" or "cumbersome and over-engineered." It seems appropriate at
the moment, though.

internal/run/help.go

@@ -1,7 +1,7 @@
 package run
 
 import (
-	"os"
+	"fmt"
 )
 
 // Help is a step in a helm Plan that calls `helm help`.
@@ -9,18 +9,25 @@ type Help struct {
 	cmd cmd
 }
 
-// Run launches the command.
-func (h *Help) Run() error {
+// Execute executes the `helm help` command.
+func (h *Help) Execute() error {
 	return h.cmd.Run()
 }
 
-// NewHelp returns a new Help.
-func NewHelp() *Help {
-	h := Help{}
+// Prepare gets the Help ready to execute.
+func (h *Help) Prepare(cfg Config) error {
+	args := []string{"help"}
+	if cfg.Debug {
+		args = append([]string{"--debug"}, args...)
+	}
 
-	h.cmd = command(helmBin, "help")
-	h.cmd.Stdout(os.Stdout)
-	h.cmd.Stderr(os.Stderr)
+	h.cmd = command(helmBin, args...)
+	h.cmd.Stdout(cfg.Stdout)
+	h.cmd.Stderr(cfg.Stderr)
 
-	return &h
+	if cfg.Debug {
+		fmt.Fprintf(cfg.Stderr, "Generated command: '%s'\n", h.cmd.String())
+	}
+
+	return nil
 }