Import the config system form ideos

[dgrelaud]

Import the config system from ideos. Change only one thing:

• Have the possibility to see all config keys with default value in the config file.
• N2 : keep default keys to avoid losing configuration when importing a DB

Possible solution (because comments are not allowed in JSON) : like in lang file, add a character at the beginning (@) and push all default config keys at the bottom of the files, in alphabetical order.