Feature Request: Provide conflicting_options and option_dependency

Why don't you provide `conflicting_options` and `option_dependency` shown in [example/real.cpp](http://www.boost.org/doc/libs/1_61_0/libs/program_options/example/real.cpp)?
It is useful.

If provided, It is desirable that `conflicting_options` is variadic.

Start by reading example/real.cpp, which demonstrates conflicting_options and option_dependency. Define the public API needed to provide both facilities, including the requested variadic form for conflicting_options, and confirm that the example’s use cases are supported; the issue names no tests, so project test coverage must be identified before implementation.

Written by the indexing model from the issue text.