title: Have pybliometrics log to file
body: <p>When a Scopus ID was merged with another one, pybliometrics logs something like this to the console:</p>
  <blockquote>
  <p>author_retrieval.py:125: UserWarning: Profile with ID 57202566304 has been merged and the new ID is 7202624351.  Please update your records manually.  Files have been cached with the old ID.
  warn(text, UserWarning)</p>
  </blockquote>
  <p>How can I have pybliometrics log its merged Scopus ID messages to a file, so I can handle them later by manually updating my Scopus ID list?</p>