Add an option to control xhprof enable conditions

[staabm]

IIRC most of this functionality is already provided in https://github.com/staabm/xhprof.io

[AlloVince]

@staabm above repo looks like just use a key value to control the profiler, my way is use a closure, it will be more flexible

[staabm]

@AlloVince the related config file is loaded using require therefore it can contain any logic you like, e.g.

return array(
    'enabled' => rand(0, 100) === 42;
);