adding null value handling in Writer class #28

In response to #28 we are introducing `setNullHandling` and `getNullHandling` methods into the `Writer` class to handle the `null` value when importing data into a CSV. the setter method will take one of the newly introduced constants:

* `Writer::NULL_AS_EXCEPTION`: `Writer::insertOne` will throw a `RuntimeException` when a `null` value is found **the default behavior**;
* `Writer::NULL_AS_EMPTY`: `Writer::insertOne` will convert `null` value into empty string;
* `Writer::NULL_AS_SKIP_CELL`: `Writer::insertOne` will filter out each `null` item found;

For backward compatibility, if the `setNullHandling` method is not use the class will behave with the `Writer::NULL_AS_EXCEPTION` mode on.