sif can now slot into unix pipelines. stdin is drained for targets when
it's a pipe (keyed off stdin's mode, not stdout), alongside -u/-f. naked
hosts are accepted and default to https://; explicit http(s) is kept,
other schemes rejected. -silent routes all banner/spinner/log chrome to
stderr and prints one normalized finding per line to stdout via
finding.Flatten, so `subfinder | sif -silent | notify` works.
