Add possibility of merge missing nested array values under the top-level key

**Original pull request https://github.com/Incenteev/ParameterHandler/pull/36 by @zeliard91**

Bug fix: no
Feature addition: yes
Backwards compatibility break: no
Tests pass: yes

With this, we can now have new multi dimensional arrays in the dist file under the top-level key parameter be merged in the generated parameter file.

The prompt is still waiting for a full YAML definition of the missing key.