perf tools: Add support to parse event on/off toggle terms

Adding parsing support for 'on' and 'off' terms within the
event syntax, like:

  # perf record -v -e #
  # 'cycles,irq_entry/on=cycles/,irq_exit/off=cycles/' ls

Only string value is accepted for both terms. The name will be used
in a search for toggle event.

Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>