Stop buffering whole Scarb stdout in MetadataCommand

[maciektr]

Problem

As of #1098 the MetadataCommand allows Scarb stdout inheritance. Unfortunately, the current implementation buffers whole output inside and prints it all at the end. For instance, if Scarb subprocess encounters an error, the current implementation will not be able to show any output messages to the user.

Proposed solution

MetadataCommand should stop buffering output and work on some kind of stream instead, passing output to stdout line-by-line, while searching for metadata json.

Acceptance criteria:

• Scarb output is passed to stdout from MetadataCommand on the fly
• output color is handled properly
• isatty is set correctly on the child process