Make TypeErrors more informative when an optional header is missing

[dean0x7d]

One of the most frequent questions, both in the issues and Gitter, is: "Why doesn't the std::vector<T> conversion work?" The answer is usually: missing #include <pybind11/stl.h>.

As far as I know, there is no way to diagnose this issue at compile time (at least not while also keeping all the dependencies optional), but this PR attempts to add an informative message to conversion errors which are likely caused by missing headers.

The note is only appended if std:: is found in the function signature. This should only be the case when a header is missing. E.g. when stl.h is included, the signature would contain List[int] instead of std::vector<int> while using stl_bind.h would produce something like MyVector. Similarly for std::map/Dict, complex, std::function/Callable, etc.

There's a possibility for false positives, but it's pretty low.

The full error message looks something like this:

TypeError: missing_header_arg(): incompatible function arguments. The following argument types are supported:
    1. (arg0: std::vector<float, std::allocator<float> >) -> None

Invoked with: [1.0, 2.0, 3.0]

Did you forget to `#include <pybind11/stl.h>`? Or <pybind11/complex.h>,
<pybind11/functional.h>, <pybind11/chrono.h>, etc. Some automatic
conversions are optional and require extra headers to be included
when compiling your pybind11 module.