support formatter URIs for external identifiers (extid)

[WolfgangFahl]

see Using_formatter_url_to_construct_links_in_SPARQL

with example:

# Get 10 paintings that have a link to RKDimages (P350)
# Use the formatter URL (P1630) to construct the links to RKDimages
#defaultView:ImageGrid
SELECT ?item ?image ?rkdurl  WHERE {
  wd:P350 wdt:P1630 ?formatterurl .
  ?item wdt:P31 wd:Q3305213 .
  ?item wdt:P18 ?image .
  ?item wdt:P350 ?rkdid .
  BIND(IRI(REPLACE(?rkdid, '^(.+)$', ?formatterurl)) AS ?rkdurl).
  } LIMIT 10

try it!