Support external definition dependencies
It would be nice if Anolis supported syntax like the following:
This document uses the term <dfn data-anolis-spec=foo>foo</dfn> as defined in the Foo Standard. A <dfn>bar</dfn> is like a <span>foo</span>, but different. A <span>bar</span> should look like a <span>foo</span>.
This would bring the external definition of 'foo' into the same scope as the local definition of 'bar', eliminating the need to use 'data-anolis-spec=foo' on each instance of 'foo' (though, obviously, that would still be supported).