perf diff: Color the Delta column

Color the numbers in the Delta column using percent_color_snprintf().

Generalize the coloring function so that we can accommodate all three
comparison methods in future patches: delta, ratio, and wdiff.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/1388388861-7931-3-git-send-email-artagnon@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>