Add a more generic hook mechanism

[luna-duclos]

This would be useful in case anyone wants to hook in something specific to their application, such as custom metrics or other stuff

[qustavo]

what about integrating it with: https://github.com/gchaincl/sqlhooks?

[luna-duclos]

Looking at it, it looks like integration isn't even necessary, I see no reason why this package and gchaincl/sqlhooks wouldn't play nicely together.

Edit: It doesn't look like it is up to date with Go 1.8

[qustavo]

@luna-duclos you're right, they should.
if you're getting problems with 1.8 can you open an issue pls?