Run validation before any commands start running

Let's say I have the following commands:

```
file-read test.json | transform | file-write test.csv
```

In this use-case, let's say I want to validate the file extension before running a command. I add the validate method, but it won't run until _after_ the whole file has been read in and transformed. In my use-case, that may take hours.

I'd like the validation to run immediately before any command starts running.

@dthree Any thoughts?