Fold multiple whitespace characters inside <dfn> to a single space.

Currently, `<dfn>new\n   term</dfn>` does not match `<a>new term</a>`.
This patch normalizes whitespace inside definition link text in the
hopes of fixing exactly that problem.
Fold whitespace characters in update.py's anchor processing.
Merge pull request #155 from mikewest/fold-whitespace

Fold multiple whitespace characters inside <dfn> to a single space.