Add support for external documentation consumption for custom checks

[borkweb]

Talking with @d4mation, it'd be great to be able to supply markdown documentation for custom checks. Kind of like:

{
  "checks": {
      "has-min-js": {
          "type": "simple",
          "file": "checks/has-min-js.php",
          "help": "checks/has-min-js.md"
      }
  }
}