MCO-472 Add a collective data plugin

Here we add a collective data plugin to allow for the membership of a
collective to be testing in the compound filter language.

Example:
    mco find -S 'collective("us").member=true'

This compliments the visibility of collective in the filter feature added as
MCO-456.
Merge pull request #242 from richardc/MCO-472-collective_data_plugin

MCO-472 Add a collective data plugin