* 6.0.0
Change Reader to functor type, breaking change.
Change Reader functor to allow any return type, but specifically need bool-testable return for NOEXCEPT use.
Change List and Map templates into template templates to enforce proper type use and to clean up user template invocations (i.e. `args::ValueFlagList<std::string, std::unordered_set<std::string>>` becomes `args::ValueFlagList<std::string, std::unordered_set>`, also breaking change.

* 5.0.0
Implemented proper subparsers.
Added better C++11 style.
Improved documentation.

* 4.0.0
Changed all wording:

ArgFlag -> ValueFlag
Counter -> CounterFlag
PosArg -> Positional

Argument now solely refers to command line arguments.
Value refers to the argument that flags or positionals can take and store.
Positional is a positional option, which contains a value.
Option refers to flags and positionals, which can contain values.
