Add support to run custom server logic

[ghost]

This adds support to run custom server logic with the help of revel/revel#879.

[brendensoares]

More interesting code. Will look deeper soon!

[brendensoares]

I think we should flesh this out in more detail before we accept this PR. There are likely many ways to accomplish this task.

[pedromorgan]

Na stuff the RFC.. any good idea..
We just stick it in developer and clearly flag as "experimental"

[brendensoares]

@pedromorgan how will we clearly flag it? How will we document how it works for the Revel docs? How will we know when it doesn't break existing features?

We can't just throw in code willy nilly. We have to make it hard to change Revel and easy to improve it. There must be a process for evaluating incoming code changes to decide if it's the right solution for Revel's goals.