[Feature] Allow Unknown Options from all sources

[joaomlneto]

For the command line parser, there is the allow_unregistered() method to not throw an exception whenever an option is not recognized.

However, this is missing from the configuration file and environment parsers.

It is possible to do work around this for both configuration files and environment, but I am of the opinion this should be handled in similar fashion to the command line parsers!

[ohois]

This feature exists. The third parameter for parse_config_file

https://github.com/boostorg/program_options/blob/develop/src/parsers.cpp#L101

Haven't tested with environment but I guess it already ignores unregistered variables.