Boost was only used for program option parsing. To avoid this huge
dependency, this commit replaces boost::program_options with cxxopts,
a header-only library with the same functionality.
cxxopts is added as a submodule, and Boost is removed from the
dependencies in the code and Travis configuration.
With C++17, optionals, an experimental language feature, were moved to
the “std” namespace. This makes C++17 mandatory and drops the now
obsolete “experimental” namespace.
