Markdown.pl: hook up fragment only link definitions

Markdown.pl: hook up fragment only link definitions

This works to hook up a fragment link to its section:

    # Section 1

    Link to [Top](#Section_1).

Make the same thing work when written like this:

    # Section 1

    Link to [Top][id].

    [id]: #Section_1

Or even like this:

    # Section 1

    Link to [id].

    [id]: #Section_1

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>