Feature: Init file: Add possibility to run user code on startup

[tinmarino]

It would be nice to get the possibility to have a user configuration (alias init) file loaded in the sandbox. As #85, this would just fasten workflow.

Ideas (inclusive):

1. A path given with a parameter -f
2. A string with -e
3. Load a file located in a specific (jupyter --data) path

I prefer 1 and 2 because less burden for both devs (arbitrarily choosing: option vs path) and users (reconfiguring the module: kernel.json vs copy and paste a file).