Extend table predicate to support multi-selection w/arrays.

Here's a traditional way to get all VPC subnet IDs for a region:

    vpc = Table.load 'maps/vpc.txt'
    mapping 'VpcSubnetIds', vpc.get_multimap(
        { :visibility => 'private' }, :env, :region, :subnet)

Now you can filter rows using array.include?, not just ==.  Eg.

    vpc = Table.load 'maps/vpc.txt'
    mapping 'VpcSubnetIds', vpc.get_multimap(
        { :zone => ['a', 'b'] }, :env, :region, :subnet)