Add first ugly support for std::function overload for v8pp::convert

[YarikTH]

• [ ] Need to add tests
• [ ] Fix forward declaration of wrap_function (Template parameter redefines default argument in "typename Traits = raw_ptr_traits")
• [ ] Make sure function converter is composable with other converters (std::function<void(std::vector)>, std::vector<std::function<void()>> etc)
• [ ] Fix problem that caller should additionally include v8pp/call_v8.hpp or template call_v8 would be illformed

Closes #142