More flexible filtering

[gmccall]

At the moment the filters are all 'and' so for example clicking ECHR and ICC will show the countries that are part of both.

For some filters it would be useful to have an 'or' filter, so you can see countries that are part of ECHR or the AU.

Even better would be the option to use either 'and' or 'or' so you could display countries which have an ICC, ICTY or ICTR case and have signed up to the Rome Statute.