Extend Drush command to show the list of changed active config

@nerdstein: can you checkout this branch: i2805369-list-of-changed-active-config? It already lists the name of changed config. (use: drush cpex config --changelist)
If there is a new config and I want to export it with the help of drush it throws error that the config doesn't exist. Can you check the export?
How to reproduce:
- I made a full export than a full import.
- I installed devel module.
- Checked the change list and received:
- devel.settings
- system.menu.devel
- core.extension
core.extension was modified and I could export with no problem. For the other two I received the error.

Ported with version 1.3.