Added a fitter_kwargs argument to k_fold_cross_validation utility function
Added a fitter_kwargs argument to k_fold_cross_validation utility function

- when doing k_fold_cross_validation, the function should be able
to pash in key-word arguments to the fitter
- for example, for the coxph model, you should be able to pass
in the strata argument
- updated k_fold_cross_validation docstring

CamDavidsonPilon/lifelines