Make the cythonize command line more useful

[netheril96]

A couple of more common build options, such as --numpy, --include_path, --libraries, --cflags, --ldflags should be added to cythonize command line so that it can be used easily for the majority of building tasks.

Setting up a setup.py script for every pyx file I need to write is really, really tedious, which put me off from cython for years before I finally tried it for the first time (and that was in IPython notebook with cython magic, so I still don't know how to write setup.py). I am sure that many beginners to cython are just scared of the complicated build process like me, and never have the chance to actually try it. Please make it more accessible.

The %%cython magic in IPython has much better sane defaults and useful command line options than the cythonize script.

[netheril96]

I extracted the cython magic into a separate script at https://github.com/netheril96/utilities/blob/master/mycython.py.