add '-s/--simulate' to run data extraction without download

Useful for quick testing (even though -g and -j kind of do the same)
and to fill a download archive without actually downloading the files.

-s does the same as the default behaviour, except downloading stuff.
Maybe it should get a more fitting name, as it does actually write to
disk (cache, archive)?

gallery_dl/__init__.py

@@ -275,6 +275,8 @@ def main():
                 jobtype = job.KeywordJob
             elif args.list_data:
                 jobtype = job.DataJob
+            elif args.simulate:
+                jobtype = job.SimulationJob
             else:
                 jobtype = job.DownloadJob