perf diff: Color the Weighted Diff column

In

  $ perf diff -c wdiff:M,N

color the numbers in the Weighted Diff column using color_snprintf(),
picking the colors using get_percent_color().

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/1388390555-10808-1-git-send-email-artagnon@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>