add support for boolean params

[alcinos]

This simple PR add supports for boolean parameters in the argument parser.

This enables to use the following kind of argument declaration
("bool_flag", dict(type='bool', default=False , help="Example boolean flag"))

Then the corresponding command line would look like:
python my_script.py --bool_flag 1

[facebook-github-bot]

Thank you for your pull request. We require contributors to sign our Contributor License Agreement, and yours has expired.

Before we can review or merge your code, we need you to email cla@fb.com with your details so we can update your status.

[facebook-github-bot]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Facebook open source project. Thanks!