Consider supporting content annotation using comments or an element

[thomas-darling]

Angular 2 supports annotating content by placing a comment before and after the content that should be exported. We should consider whether adding a similar feature to this plugin would be a good idea.

[thomas-darling]

See related suggestion in #5.
If we do decide to add a non-attribute way to annotate content, a <translate> element might be a nicer way to go, despite the possible confusion. We would definitely need validation to ensure users don't accidentally add e.g. class names to such an element, as it might be removed during localization...

[rs3d]

Maybe we can provide an option to keep, replace or delete (default) the element if someone wants to.
It could be a nice feature to make it possible to use a custom element (eg. for web components: https://www.html5rocks.com/en/tutorials/webcomponents/customelements/) and add multiple attributes to it.