Describe `Program` validation strategy

[dogonthehorizon]

This should probably go in the wiki as a design doc of some sort. Some discussion with @hibes surfaced the fact that we haven't done a good job communicating some of the design decisions made in the project.

This document should describe the goals we have in mind for SignalFlow program validation so that we can provide great error messages to users before they ever hit the SignalFx API.

This document may also include a strategy for implementing future validations.

[dogonthehorizon]

Provided some brief thoughts on why we want to do Program validation in the library:
https://github.com/Nike-Inc/signal_analog/wiki/Developers-::-Program-Validation

Creating a few tasks to implement the sample validations on that page.