AddRepo step that calls helm repo add [#26]

As with some of the other commands, I'm not sure `--namespace` is
relevant here. Just rolling with the "at worst it doesn't hurt" theory.

internal/run/addrepo.go

@@ -0,0 +1,48 @@
+package run
+
+import (
+	"fmt"
+)
+
+// AddRepo is an execution step that calls `helm repo add` when executed.
+type AddRepo struct {
+	Name string
+	URL  string
+	cmd  cmd
+}
+
+// Execute executes the `helm repo add` command.
+func (a *AddRepo) Execute(_ Config) error {
+	return a.cmd.Run()
+}
+
+// Prepare gets the AddRepo ready to execute.
+func (a *AddRepo) Prepare(cfg Config) error {
+	if a.Name == "" {
+		return fmt.Errorf("repo name is required")
+	}
+	if a.URL == "" {
+		return fmt.Errorf("repo URL is required")
+	}
+
+	args := make([]string, 0)
+
+	if cfg.Namespace != "" {
+		args = append(args, "--namespace", cfg.Namespace)
+	}
+	if cfg.Debug {
+		args = append(args, "--debug")
+	}
+
+	args = append(args, "repo", "add", a.Name, a.URL)
+
+	a.cmd = command(helmBin, args...)
+	a.cmd.Stdout(cfg.Stdout)
+	a.cmd.Stderr(cfg.Stderr)
+
+	if cfg.Debug {
+		fmt.Fprintf(cfg.Stderr, "Generated command: '%s'\n", a.cmd.String())
+	}
+
+	return nil
+}