Added new option --write_items_since to write items to either rsswriter or htmlwriter created in the last x minutes. RSS file uses now as default, the past 360 minutes (6 hours) instead of the past 200 records. In some feeds this could generate overly large results that the feed client would have already polled in a previous run.

 Created new Abstract Class in crawlerbase, which crawlers should extend.

 New API added to check if a record is to be rejected based on its body. This is useful to drop items that we don't care based on contents of its page body that can't be seen in the overall search list