Allow setting of filter lenght to specify the number of rightmost digits

I have made a tiny addition to the LDAP module, that allows you to set a filter length specifying the number of rightmost digits to check for a match.

E.g., if it is set to 9, it would also find +31612345673 in the LDAP, while the original caller-ID is 0612345673.

If the setting is left empty, it will take the entire CID and compare it with a number in the LDAP.

I hope this is a useful contribution.
Thanks!