I'd like to make a contribution by implementing the FILTER clause, as better described in these blog posts:
https://medium.com/little-programming-joys/the-filter-clause-in-postgres-9-4-3dd327d3c852#.r492befze
and
https://blog.jooq.org/2014/12/30/the-awesome-postgresql-9-4-sql2003-filter-clause-for-aggregate-functions/

I'm using this, and WITHIN GROUP as well, to build up elaborated reports. Currently I'm using literal SQL, but have started something here, but any help or advice would be nice.