Make configurable whether to generate bindings for referent/pointer return type without return_value_policy specification

[jslee02]

This is a restriction for Boost.Python, but not for pybind11 as pybind11 uses default policy when not provided (see return_value_policy::automatic[_reference] in this doc).

For example, pybind11 allows to generate bindings for getName() with no return value policy specification:

struct Pet {
    Pet(const std::string &name) : name(name) { }
    void setName(const std::string &name_) { name = name_; }
    const std::string &getName() const { return name; }

    std::string name;
};

Possible solution would be introducing a tag in options to specify default return value policies for pointer, const pointer, reference, and const reference. This way we could reduce the number of return value policy specification per types (see this example).