`EXPLAIN`, `EXPLAIN ANALYZE` support

[vemv]

Hi!

It would be nice to support https://www.postgresql.org/docs/9.1/sql-explain.html OOTB.

I think that EXPLAIN doesn't belong to the SQL standard to this would be an extension to honeysql.

Cheers - V

[seancorfield]

Interestingly, this is something that clojure.java.jdbc supports (for nearly all databases) that I haven't yet added to next.jdbc. It's definitely not PG-specific.