Document top-level API a little, make testing from the command line e…

…asier. (facebookresearch#106)

Summary:
Pull Request resolved: facebookresearch#106

Add docstrings and types to a couple functions exported to the top-level PyText API.

Update the test cli mode to take in a PyTextConfig rather than a TestConfig for easier command line usage.

Reviewed By: ahhegazy

Differential Revision: D13367488

fbshipit-source-id: 2347e63f4e31a737566ef6aabd3399a79e6ff023