Specify static completion material.

Offhand, it seems like completion and validation could be defined together. Completion feels like it should come after the program, validation feels like it should come before. If you can do both with the same definition, then maybe there is a function that extracts the completion material and uses it in the program.

```
arguable.validateUsingCompletion()
arguable.validate(require('arguable/completion'), arguable.completion)
```

Something like that. Verbosity doesn't bother me if it gets a lot of work done.