Wrong 'require' for custom helpers #12

It seems like there is a bug in code which allows to use custom helpers. I suppose this config codecept.json was planned to work nice

{
  "helpers": {
    "MyCustomWebdriver": {
        "require": "my-custom-webdriver"
     }
  }
}

but now it seeks require key in main helpers config.

Also, it would be nice if require path here is resolved relatively to codecept.json location, like

{
  "helpers": {
    "MyCustomWebdriver": {
        "require": "./lib/my-custom-webdriver"
     }
  }
}

because now it will work only with

{
  "helpers": {
    "MyCustomWebdriver": {
        "require": "./../../../lib/my-custom-webdriver"
     }
  }
}

I am ready to make PR, but I wonder why there are no tests here, are they not written yet, because I feel wrong to write PR without tests 😃