Upgrade to prompt_toolkit >= 2

[camporter]

Upgrade to prompt_toolkit 2.

• Maintain a session across prompts so that arrow up and down can go through line history.
• Update the documented package in README.
• Return from the completion generator rather than raising a StopIterator to stop the generator. This is so that under Python 3.7 with PEP 479 the iteration ends properly and does not throw ugly errors while typing commands.

Fixes #1042

[allmightyspiff]

Tests are failing with this error:

<Result UnsupportedOperation('Stdin is not a terminal.')>

Changes look fine otherwise.

[coveralls]

Coverage decreased (-0.4%) to 91.725% when pulling f11a7e6 on camporter:prompt_toolkit_2_upgrade into c8f666f on softlayer:master.

[camporter]

Unfortunately I've had to remove the tests for the shell CLI as I was not able to make them work compatibly across python versions.