Add support for comments

[mgeisler]

We should teach mdbook-xgettext to react to comments in the Markdown. Comments could be used for:

• Marking the next paragraph or code block as non-translatable. This could save translators a lot of work if there are many code blocks which shouldn't be translated because they don't have any strings or comments.
• Add translator comments. Gettext supports comments on each message and xgettext extracts them using a special syntax (which we should try to replicate).
• Add a message context. This is again s Gettext feature: the same message can appear multiple times, but with s different context each time.

[mgeisler]

I somehow submitted this twice, let's close this for #63 which has a slightly better title.