We could either implement simple chaining for variable maps, or
implement generic composition classes. The former was choosen,
mostly because of simplicity.

There were two implementation approaches for multiple option
occurrences in options_and_arguments. First is store them
separately. The advantage is that it's easy to obtain all
occurrences before certain position on command line. The
disadvantage is that we cannot return a reference to 
vector<vector<string> > in get_all_values. It was considered
that if support for position-dependent options is to be
added, then we're be mostly interested in occurrences of
a single option that were before some point. That's possible
with vector<vector<string> > storage.