Implement preprocess option in the testling field

[tarruda]

This option should make simple perform complex transformations in the bundle for testling. For example:

{
  "testling": {
    "preprocess": {
      "command": "./bundle.sh",
      "args": ["arg1", "arg2"]
    }
  }
}

[tarruda]

If you want I can squash everything into a single commit

[ljharb]

Looks like #85 added the preprocess option; i'll rebase this and retain what i can.

[ljharb]

@tarruda i've rebased this; and i know it's been many years since you made the PR :-) I just took over this package and don't yet have full context. Can you help me understand what changes remain, and how they could be tested and made to work?