Organize checks

[mcandre]

As a developer, I want stank to move most logic out of applications, and into libraries, so that I can build more complex systems based on the stank suite.

• [ ] Move checks out of the application logic.
• [ ] Design the API of checks so that they operate on semi-populated stank.Smell's, constructing os.File handles, bufio.Reader's, etc. as needed.