add a recs-substream that emulate's Perl's range operator

Emulates Perl's bistable string range operator in recs.  One main use for this is truncating everything past a certain date in very large log files.

"Up-ported" from a recs 3.3 version I wrote; I think I got all the little bits that have changed right.  I'd be interested in any nitpicks about style, --begin/--end naming, and whether or not --end should be inclusive or exclusive. :)