Add type hints

Optional type hints allow static type checker (for example [mypy](http://mypy-lang.org/), [pyre](https://pyre-check.org/), [pyright](https://github.com/microsoft/pyright), many IDEs) to check validity of code and allow Python programmers to write less buggy code. `jmespath` is one of the most popular libraries according to PyPI stats. I believe that adding type hints/annotations here will evolve entire Python ecosystem.

It can be added natively since Python 3.5 but also can be added in external stubs (`*.pyi` files) or commentaries (`# type: int`). Both methods will work with Python 2.7 and Python 3.4.

What do you think?