Support recursing into `{{#include foo}}` directives

[mgeisler]

Currently, we use mdbook-xgettext as a renderer, e.g., as an output format which sees the Markdown files after all of the preprocessors have run. To match this, we recommend installing the mdbook-gettext preprocessor with

[preprocessor.gettext]
after = ["links"]

This means that the mdbook-gettext preprocessor sees any files included via the built-in links preprocessor, just like the mdbook-xgettext renderer does.

This works well, and with #95 fixed, we even minimize the strings extracted this way. However, we could improve on this a bit by handling the include directives ourselves (hopefully by calling out to the links preprocessor). The advantage of this would be

• Correct source lines for the translations: right now the source line points back to the Markdown file, but with a post-expansion line number.
• Less updates of line numbers: if a Markdown file includes a.txt first and then b.txt, then a change to a.txt will disrupt the line numbers of all following messages. This causes unnecessary churn in the translation files.

To implement this, we should instead run mdbook-xgettext as a preprocessor, and run it before the links preprocessor.

This might of course not be worth it, especially as links.rs doesn't seem to expose it's functionality.

[mgeisler]

Having thought a bit more about this, it might not be as easy or important as I first thought.