Additional flags to pass to underlying MPI libraries

[roryck]

As requested by Jim / Jian

Please add the capability to pass additional flags from mpibind to the underlying MPI libraries. The first examples would be the cray-mpich specific flags --label --line-buffer, but a general solution is desirable as well.

[roryck]

As a general goal of mpibind is transparent opperation between MPI libraries I would like to not introduce anything that a user needs to change when switching MPI within a script. As a prototype I'm going to incorporate the cray-mpich flags as recognized options, and translate or ignore them within other MPI libraries depending on native support for similar options.