Remove satisfied `TODO`.
Fix broken tag attribute test.

The tag attribute test needs to assert that the `$attributes` hash is
not present, but it instead asserts that `$attr` is not present.
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`.
Docco on includes map structure.

Added Docco to remind myself why the `includes` map also contains
aliases that map the namespace URI mapped by `xmlns` to the URL of the
included library.
Remove debugging output.
Sub-attributes and frame management.

Implemented sub attributes, the ability of a sub tag to reference the
attributes of the tag that defined it.

With this commit, we begin to do some frame management, maintaining a
state that is consistent across invocations of sub-tags. In a subsequent
commit we will ensure that an included tag does not see the context of
the caller, that it has it's own clean context inheriting the context of
the `include` element that defined the tags.

Closes #86.
Merge branch `tag-attributes` into `master`.