Add a .mailmap to canonicalize commit names/emails
Update and make configurable the default format

- Rather than http://zephyr.1ts.org/wiki/df, use http://mit.edu/df, which is
  shorter
- Rather than a bare URL, prefix "Config error: see " to make it clearer what
  is going on
- Make the default format configurable using ./configure, to aid users with
  particularly severe length constraints