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

[Pym]

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.

[luxifer]

:+1: please merge this

[luxifer]

Any chances to have the PR merged?

[stof]

I will dedicated some time this weekend to review it

[elvisciotti]

this feature would be nice to have

[stof]

I think this breaks things for cases where you have a list in an argument (for instance a list of trusted proxies, even though that's the bad example for Symfony projects as trusted proxies are needed earlier than the loading of parameters)