Suggestion: Provide an include_headers() method in Python when installed with pip

[ashyantony7]

For projects involving python wrappers to C++ libraries, it is useful to have the include header locations.

Currently, when spdlog is installed with pip, the headers are located in the root (spdlog.h, null_mutex.h etc..)
It would be really useful to have these headers organized in the way they are intended and a method is available in Python to get the include location.

For instance, Pybind11 provides a method (pybind11.get_include()). This is especially useful for having a setup.py for compilation.

[ka-bo]

Once the installation is fixed (see #20) including this option might be a good idea.