Implemented quoting of object keys (fixes #1).

Like suggested in issue #1, I added another config option ``normalizeObjectKeys`` which can be set to ``true`` (defaults to ``false``) and which converts every object key into a string and applies the usual quote transformation logic of esformatter-quotes, too.

The diff looks probably bigger than it is, because of the test files and because of the refactoring of the actual transform logic, which is still the same, but was put into a ``function`` to make sure it works the same in the usual String case and in the ObjectKey case, too.