Separate optional and variant support from other STL containers

I'd like to propose that we:

• move std::optional and std::variant conversion support, which are currently in pybind11/stl.h, to a new file (say pybind11/utility.h since they are in the STL utility library in C++17).
• include pybind11/utility.h from pybind11/stl.h so that we maintain full backward compatibility.
• update the docs Overview page to mention the new header.

My motivation for this proposal is that the conversion for other containers (set/vector/map) in pybind11/stl.h comes with a major downside, documented in http://pybind11.readthedocs.io/en/stable/advanced/cast/stl.html#automatic-conversion, and I believe that optional/variant can be useful in their own right.

If you like the idea, I'm happy to prepare a pull request. Please let me know. #

Start in pybind11/stl.h and locate the std::optional and std::variant conversion support. Move that support to the proposed pybind11/utility.h, include the new header from pybind11/stl.h for backward compatibility, and update the documentation Overview page to mention it. Done means the support is separated without breaking the existing header interface and the docs describe the new header.