Feature request: set those args which are matched to 0

[pfeatherstone]

In args::parse_args, rather than parsing const char** argv , parse char** argv and set those argv entries to null if they are matched.

Furthermore, in ParseCLI, only populate the std::vector<std::string> args vector with entries in argv which are not null.

With both suggestions, we can start nesting parsers. For example we parse a first set of flags, then go to the next set of flags depending on the result of the first parser, using the remaining non-null argv arguments.