Autolink: Update DOI regex
Autolink: Update DOI regex

* The assigned registrant code is always at least 4 digits long. The
  comment which enticed me to catch 3 digit codes was later corrected.

Comment about non-change:
* While < > are valid characters, so could potentially be removed from
  the negative look-ahead, the chance to match HTML markup is too risky.

mdmower/doi-resolver-chrome,mdmower/doi-resolver-chrome,mdmower/doi-resolver-chrome