Add membership operator for query

[luqasz]

Use case:
Select all arp table entries where entry's interface belongs to a list of interfaces.

possible example:

address = Key('address')
interface = Key('interface')
api.path('/ip/arp').select(address, interface).where(interface.In('ether1', 'ether2')):

[luqasz]

Added in https://github.com/luqasz/librouteros/tree/3.1.0