Fetch top lists matching this keyword, then return Twitter screen
    names along with the number of different lists on which each appers..

def fetch_exemplars(keyword, outfile, n=50):
    """ Fetch top lists matching this keyword, then return Twitter screen
    names along with the number of different lists on which each appers.. """
    list_urls = fetch_lists(keyword, n)
    print('found %d lists for %s' % (len(list_urls), keyword))
    counts = Counter()
    for list_url in list_urls:
        counts.update(fetch_list_members(list_url))
    # Write to file.
    outf = io.open(outfile, 'wt')
    for handle in sorted(counts):
        outf.write('%s\t%d\n' % (handle, counts[handle]))
    outf.close()
    print('saved exemplars to', outfile)