Finished the addition of the Json parsable output with the -p flag.
To do this I had to:
- Modify LTL::PrettyPrinter to get a generic ostream instead of
  working always with std::cout
- Adding a lot of exports in the leviathan.hpp header files.
  make_*** functions are not enough if one needs to use the AST from
  the outside. isa<>, fast_cast<> and all the types also need to be
  exported.