Forking mode should accept argument to specify the correct MPI distribution

[connorjward]

Currently in forking mode we inspect the output of mpiexec (soon to be mpi4py, #29) to figure out which MPI distribution we use. From #27 we know that this isn't a totally robust solution. This is basically lifted from #16 but I propose that we let pytest take an argument to specify the MPI distribution in use. Something like:

$ pytest --mpi-impl mpich
$ pytest --mpi-impl msmpi
$ pytest --mpi-impl openmpi