Support system commands for include/link flags

[molpopgen]

It'd be handy for the cell magic to support something like this:

%%pybind11 -I=`gsl-config --cflags` -I=`python -c "import numpy;print(numpy.get_include())"`

When using other Python packages that provide headers, it'd be great not to have to hard-code in the linker paths.

Would such a thing be possible?