Using non-default version of python with hydrogen

[iprokin]

Taking opportunity, I thank you for your work on amazing hydrogen.

I have both python2 and 3 jupyter kernels installed. On ArchLinux the default python is python3 with which hydrogen runs nicely. However, I often want to run it with python2 kernel. I didn't find an easy way to select another python kernel in hydrogen.

None of the things I tried worked:

1. "Language Mapping" {"Python":"python2"} in hydrogen's settings.
2. running atom with "PYTHON=python2.7 atom"
3. running atom with "alias python='/usr/bin/python2.7'; atom"

It would be nice to have an easy switch between python 2 and 3.

[slavaGanzin]

Close when this would be merged: #138

[slavaGanzin]

@iprokin Try changing kernels with Hydrogen/master