ENH: Add Lookup machine-task example train file

[gautierdag]

This adds machine_task_lookup_example.py which is an example of how to train a task in machine-task using machine.

Example for how to run file:

python machine_task_lookup_example.py --default_settings "baseline_2018"

This uses the baseline parameters stored in machine-task found by Hupkes et al. 2018

python machine_task_lookup_example.py --default_settings "Hupkes_2018"

This makes the training use the final settings used in Learning compositionally through attentive guidance(Hupkes et al. 2018)

NOTE: Must have pulled the machine-task and machine repos into the same folder. Since this script adds machine-task to the path and assumes that machine-task is present one directory above.

See PR in machine-tasks for new .yml file: i-machine-think/machine-tasks#25