Check for invalid fields

[xjamundx]

I think the biggest thing you want to check if is people are using unsupported field names. This is true both at the root level and inside nested objects.

[kentcdodds]

If we get https://github.com/kentcdodds/configuration-validator/issues/1 supported, then all we need to do is create basic validators for at least every root property and that will cover us for now. Even something like:

export default {
  key: 'devtool',
  validate() {}
}

Would be sufficient for now. Then we can go back through and fill in actual validation.

[kentcdodds]

This will be closed by #13