Add customisation

Add some customisation options via WordPress Filters

We can now filter the display of CMB options in the backend via the following example:
```
function set_pico_defaults( $defaults ){

	$defaults['custom_meta_options']['box_1'] = false;
	$defaults['custom_meta_options']['box_2'] = false;
	$defaults['custom_meta_options']['video'] = false;

	return $defaults;
}

add_filter( 'pico_slider_set_defaults', 'set_pico_defaults' );
```

or if your game, you can edit the $defaults var directly in the plugin...

@BronsonQuick Not sure if this is the best way, happy to refactor with a better approach.