Python framework from streaming subprocess output using asyncio.

Features include:

• Launching child, processes, while keeping your parent process responsive.
• Callbacks when output it printed by the child processes and on other significant events.

This code was adapted from an example originally posted by Kevin McCarthy: https://kevinmccarthy.org/2016/07/25/streaming-subprocess-stdin-and-stdout-with-asyncio-in-python/ See his post for more details about the use of Asyncio.