perf script: Add comm filtering option

Allows collecting events system wide and then pulling out events for a
specific task name(s). e.g,

    perf script -c gnome-shell,gnome-terminal

Applies on top of:
    https://lkml.org/lkml/2011/11/13/74

v2->v3
- update Documentation

v1->v2
- use comm_list from symbol_conf

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1321894972-24246-1-git-send-email-dsahern@gmail.com
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>