Adjust command hiding to use similar convention as hidden flags

plus breaking out "setup" portion of `App.Run` into its own method, cleaning up
some bits of the help templates, and allowing for runtime opt-in of displaying
template errors to stderr.

app_test.go

@@ -1124,8 +1124,8 @@ func TestApp_Run_Categories(t *testing.T) {
 	output := buf.String()
 	t.Logf("output: %q\n", buf.Bytes())
 
-	if !strings.Contains(output, "1:\n    command1") {
-		t.Errorf("want buffer to include category %q, did not: \n%q", "1:\n    command1", output)
+	if !strings.Contains(output, "1:\n     command1") {
+		t.Errorf("want buffer to include category %q, did not: \n%q", "1:\n     command1", output)
 	}
 }