Optional memory release code for modules

[holzschu]

Currently, modules created by pybind11 have no m_free function. For most applications, it works fine, and in some cases having an m_free function would actually cause a crash (I've read the discussion in some PR that introduced an m_free function). But other applications (for example the one I am developing, Carnets) need to clean up memory and re-initialize variables when leaving Python.

Cython has solved the issue by providing an option (generate_cleanup_code). By default it is set to 0 (no cleanup code), but if it is set to a value larger than 3, Cython generates extensive cleanup code for everything. Would it be possible to have a similar option for pybind11?