Multiple Default Providers

[mkarg]

Sophisticated CLI tools allow the provision of multiple default sources, e. g. in a first step these tools will check environment variables, and if nothing found, they will check property files. It would be a very beneficial addition to jCommander to be able to register an ordered sequence of IDefaultProvider which will be checked, and the first default to be applied.

[mkarg]

@cbeust WDYT? If we will provide this, would you accept it?

[cbeust]

Sure, sounds useful!