feat: add "instrument" option as alternative to specifying noop instrumenter

[bcoe]

it felt a little weird providing a noop instrumenter directly in __coverage__, this pull introduces the instrument flag instead -- making it possible to setup __coverage__ with the following config:

 "nyc": {
    "include": [
      "index.js"
    ],
    "require": [
      "babel-register"
    ],
    "reporter": [
      "lcov",
      "text"
    ],
    "sourceMap": false,
    "instrument": false
  }

which feels cleaner.

[bcoe]

@dtinth added the --instrument flag which, in my opinion, makes configuring __coverage__ feel less weird.