Disable output buffering

The performance benefit of buffering is minimal, the blocking behavior on piping
to a secondary program is annoying.

Test set-up:
- "diff" is a file produced from `git diff` in the ACPICA repository and
  consists of 965214 lines.
- Command: time `colordiff < diff > /dev/null`

Before this patch:

    real    0m2.602s
    user    0m2.584s
    sys     0m0.016s

After this patch:

    real    0m3.470s
    user    0m3.048s
    sys     0m0.416s

As you can see, there is a difference, but given that there are likely other
limitating factors (tty speed, input buffering, disk speed), this increased time
can be ignored. With small diffs, there is even no noticable difference.