Add the ability to capture live, non-blocking stdout and stderr for executed commands

[aydwi]

Barrel can retrieve the stdout/stderr dump for a brew command after it has finished executing, but it should also have the ability to retrieve/poll the output of a command as it is executing.

There are some use cases where this feature can be extremely useful:

1. Many brew commands can take a long time to execute, so live output can help keep track of what's going on.

2. One might not always want to store a large output buffer.

3. It would be nice to have a GUI tool using Barrel be able to display the output of a command as it executes, instead of dumping at once after several seconds. Cakebrew, the most popular Homebrew GUI is unable to do this at the moment.