Added is_paying_customer() to easily check if a user is a WC customer:

Usage:

`
$customer = new WC_Customer();
$out      = 'Paying customer ? ';

if( $customer->is_paying_customer( 10 ) ) {
	$out .= "yes";
} else {
	$out .= "no";
}

echo $out;
`
Optimized is_paying_customer()
Removed break line and space
Merge pull request #5273 from corsonr/master

Added is_paying_customer() to easily check if a user is a WC customer