Automatically ignore codeblocks that don't have lit-strings or line comments

[dyoo]

This tries to automatically omit codeblocks that don't have what looks like translatable content. For the moment, we check whether the body of a codeblock contains a """ or "//" substring, which is a heuristic for checking for literal strings or line comments.

[mgeisler]

Very nice! I was somehow thinking that skipping a code block would use the new mdbook-xgettext: skip comments, but I guess there's no good reason to inject those comments just to take them out a split second later.

[mgeisler]

I think we should consider making this configurable at a later point. I can imagine a few settings:

• skip all code blocks
• skip code blocks matching a given language (the info string following the ```)
• skip code blocks where the content matches a given set of regular expressions (such as "\"" and "//" used above)

By default, code blocks would be extracted and we should probably only add the settings we need.

Let me merge this now, then we can make a new release and test it on Comprehensive Rust for a bit.