New Filter: pmpro_custom_advanced_settings

Add new custom fields to Advanced Settings with an array of options. Supports 'text','select', and 'textarea' as field_types.

Example:
```php
function my_advanced_settings() {
    $custom_fields = array(
        'field1' => array(
            'field_name' => 'test1',
            'field_type' => 'text',
            'label' => 'label1',
            'description' => 'description'
        ) ,
        'field2' => array(
            'field_name' => 'tes22',
            'field_type' => 'text',
            'label' => 'label2',
            'description' => 'another descrioption'
        )
    );

    return $custom_fields;
}
add_filter('pmpro_custom_advanced_settings','my_advanced_settings');
```
Available options:
```php
'field_name' => 'string',  // "id" and "name" attributes
'field_type' => 'string', // type of field
'label' => 'string', // field label
'description' => 'string' // description, shows up under field
'value' => 'string' //default value
'options' => array() // array of options (for select fields)
```

These values can then be used anywhere like so:
```php
$test1 = pmpro_getOption('custom_test1');
```