Allow user to customize elm-make command

[abingham]

They should be able to specify the elm-make command-line, while still providing reasonable defaults. We need to figure out the proper way to do this in the Jupyter framework. Perhaps in the profile information or something.

[abingham]

I see that if I access self.config in the kernel, it seems to reflect the contents of ~/.ipython/profile_default/ipython_config.py. Can we make this work with, say, ~/.elm-kernel/config.py?

[datakurre]

Also kernel.json could set environment variables for the kernel https://ipython.org/ipython-doc/3/development/kernels.html#kernel-specs