Add getCommandOutput

[simenandre]

Hello 👋

This pull request fixes #769 which is about making it easier to get the output from a command.

Few things we probably want to handle before landing:

• I didn't add a way to retrieve the exit code, do we want that?
• If we want exit code, how? As an object (e.g. { exitCode: number, stdout: string })?

[luketomlinson]

hi @cobraz, this is now implemented in #814!