Call helm dependency update when so instructed [#25]

As with Lint, I have no idea whether the --namespace flag actually
matters here. I don't think it will hurt, though!

internal/run/depupdate.go

@@ -0,0 +1,44 @@
+package run
+
+import (
+	"fmt"
+)
+
+// DepUpdate is an execution step that calls `helm dependency update` when executed.
+type DepUpdate struct {
+	Chart string
+	cmd   cmd
+}
+
+// Execute executes the `helm upgrade` command.
+func (d *DepUpdate) Execute(_ Config) error {
+	return d.cmd.Run()
+}
+
+// Prepare gets the DepUpdate ready to execute.
+func (d *DepUpdate) Prepare(cfg Config) error {
+	if d.Chart == "" {
+		return fmt.Errorf("chart 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, "dependency", "update", d.Chart)
+
+	d.cmd = command(helmBin, args...)
+	d.cmd.Stdout(cfg.Stdout)
+	d.cmd.Stderr(cfg.Stderr)
+
+	if cfg.Debug {
+		fmt.Fprintf(cfg.Stderr, "Generated command: '%s'\n", d.cmd.String())
+	}
+
+	return nil
+}