Add an option so we can choose to only show the untranslated strings

This might have been mentioned already, but here I go: Please add an option so we can choose to only show the untranslated strings. This should be easy since you already use different CSS-classes for translated and untranslated strings.

PS! I know I can add "display: none" to "#l10n-client .string-list li.translated" in l10n_client.css, but I want to reduce the size of the HTML too.