extend pipe size if possible

[oech3]

> truncate -s 1PB /tmp/huge
$ target/debug/fcat-main /tmp/huge|pv>/dev/null
^C31GiB 0:00:01 [7.31GiB/s]
$ target/debug/fcat-PR /tmp/huge|pv>/dev/null
^C.8GiB 0:00:01 [15.8GiB/s]
$ uutils-cat /tmp/huge|pv>/dev/null
^C.4GiB 0:00:01 [38.4GiB/s]

Not sure why this is still slower than GNU or uutils