Config file shouldn't allow kebab-cased keys

```shellsession
$ prettier -v
1.10.2
$ echo 'foo(bar, baz)' > file.js
$ echo '{ "print-width": 3 }' > .prettierrc
$ prettier file.js
foo(
  bar,
  baz
);
```

Expected:

```
foo(bar, baz);
```

as well as a warning to stderr about unknown property.

Maybe this was a regression in an CLI refactor PR.  