Support for a Configuration File and other alternative sources

Many applications can indeed be built by having explicit command line arguments, especially when a number of arguments is a small number, but when the number of arguments is bigger there might be a temptation to carry, end especially distribute, those in a Configuration File instead of supplying as individual arguments.

Would be good if the library could provide an implementation or an abstraction to merge application's arguments from various places like, Command Line, Configuration File, ... This would mean the library could apply all its beauty, like a help, required parameters check and many others, to all those configuration sources.

Have you considered extending the library to support these scenarios? I understand this would mean the library cannot be called anymore "Command Line Parser" but at the same time I think those above requirements would require the core library parsing/error reporting/... changes to organically support multiple different configuration sources.

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