Adds "admin_settings_print_peekers" hook

This pull request adds ~ 0.01 seconds to the setting page load times, but is a valuable change for plugin developers wishing to add their own setting peekers.
I am aware that there will be some changes to this function for the jQuery conversion, but the read code hasn't been changed at all. When this is converted to jQuery there shouldn't be a problem for the developer.

To make the javascript output code neater you could do the following replacement, but it adds on an extra 0.004 seconds to the load time.
Replace:
````
	$setting_peekers = '';
	foreach($peekers as $peeker)
	{
		$setting_peekers .= $peeker."\n";
	}
````
With:
````
	$setting_peekers = '';
	$i = 1;
	foreach($peekers as $peeker)
	{
		if($i !== 1)
		{
			$peeker = '			'.$peeker;
		}

		$setting_peekers .= $peeker."\n";
		$i++;
	}
````