Allow hyphens in file names.

Closes #88.
Tidy; wrap ridiculously long lines.
Renamed `library` document property `tags`.

The `library` property was hard to discern from the `library` parameter
to `rewrite` that contains the included documents. Probably ought to
rename `library` to `includes` as well.
Rename `library` parameter to `includes`.

Renamed the `library` parameter to the `rewrite` function to `includes`.