Setting up environments before loading subkernels.

[BoPeng]

This is an extension to vatlab/sos#695 and is related to vatlab/sos#688

On a multi-user environment, there can be multiple versions of the same interpreter that are loaded on demand with commands such as module load. Although users can add these commands to shell configuration files such as .bashrc, these files might not be executed when the Jupyter server is started remotely from JupyterHub. Also, using .bashrc limits users to a specific version of interpreter.

It therefore makes sense to define customized language modules (e.g. R3.3, R3.4) and associate them with certain commands (e.g. module load R-3.3 or PATH='/path/to/R3.3:$PATH) so that the correct interpreter could be located before the kernel is started.

Implementation wise, instead of the default

subkernel name: R
language module: sos_R.kernel.sos_R
interpreter: what ever R in the path
kernel: ir

we could add in user or site configuration files multiple subkernels like

subkernel name: R-3.3
language module: R (or sos_R.kernel.sos_R)
environ:  path and variables, or commands to execute
kernel: ir