Additional "processing instruction" type

In a [project I've been hacking on](https://github.com/coleifer/greendb), a redis-like frontend for LMDB, I implemented and extended the Redis protocol with a few additional types.

One of these I think may interest you I'm calling a "processing instruction". It allows the client to send additional metadata along with a single command. Currently I am only using this functionality to allow the client to specify a different database for a single command (LMDB, like Redis, supports multiple databases within a single environment). So rather than having the client do:

* SELECT 0
* do some operation on db 0
* SELECT 15
* do some operation on db 15
* SELECT 0 -- reset the client state.

I can just explicitly SELECT 0 once, and when I need to run a single command against DB 15, I send (along with the command) a processing instruction that indicates the DB for that command should be db 15:

* do some operation on db 0
* (include processing instruction to use db 15) + do some operation on db 15
* do another operation on db 0

Another usage for processing commands could be to set/modify the expiration time on the key being operated on.