Add filters to have custom columns on CSV export

[cfaria]

Hi @loureirorg,

can you consider merging this PR to the next plugin version?

I've added two filters:
uewm_custom_columns
uewm_custom_column_value_xxxxxxxx

Then we can hook them to add custom values to the CSV export for each user.

I personally use them to have data that it's not as user meta. For instance, I use the WooCommerce Subscriptions plugin and this allows me to have a column with the subscription status for each user. Sth like this:

add_filter('uewm_custom_columns', function( $columns ) {
    $columns[] = 'subscription-status';

    return $columns;
});

add_filter('uewm_custom_column_value_subscription-status', function( $value, $user_id ) {
    if ( wcs_user_has_subscription( $user_id, '', 'active' ) ) {
        return 'active';
    }

    return '-';
}, 10, 2);