Let documents {attach} static files. Refs #1019.

Until now, making static files end up in the same output directory as an article that links to them has been difficult, especially when the article's output path is generated based on metadata. This changeset introduces the `{attach}` link syntax, which works like the {filename} syntax, but also overrides the linked file's output path with the directory of the linking document.

**Important:**
This changeset builds upon changes introduced by pull request #1509, so that one must be merged first.