Support for Prepared and Procedural Statements

First, thanks for this amazing library.

Wondering if there are any plans on the roadmap to support **prepared statements**.
Here's the documentation for Postgres, but I believe MySql and most DB's support it
[Postgres Prepared statements](https://www.postgresql.org/docs/10/static/sql-prepare.html)

I am also wondering if **Procedural statements** could be supported. I'm guessing this would have to be done in DB specific way. I think this fits in with the Custom Queries support, as sometimes I may want to leverage calling a ```PG/SQL``` function that contains Postgres logic that Beam doesn't yet support. One example is if I have Postgres with PGCrypto and I want to call a function that runs ```crypt```