Listen for hooks firing within set bounds

[crstauf]

Implement #405.

Sample test code:

add_action( 'init', function() { do_action( 'qm/listen/start', 'test' ); }, 2 );
add_action( 'init', function() { do_action( 'qm/listen/stop',  'test' ); }, 6 );

[crstauf]

@johnbillion How can I forego not having the new panel included in the menu?

https://travis-ci.org/johnbillion/query-monitor/jobs/497391703#L593

[crstauf]

How can I forego not having the new panel included in the menu? ...

@johnbillion

[crstauf]

Closing in favor of #792.