Passthrough

I implemented a --passthrough option (#212). If ag is piped into and it is set, all lines are printed and matches highlighted.

Not sure about the name, ack uses "passthru" which is really ugly imho. But changing it to "passthrough" might lead to confusion as it sounds the same. Maybe there is a better third option.

Also the testcase I added fails, which is weird, because running those exact commands manually works. It looks like cram messes with stdin detection.