it should be possible to capture stdout or stderr to a file

[BrianHicks]

When implementing #7, @celsobonutti found that it's pretty inconvenient to capture stdout as a build result: you need to spawn a shell and do input redirection. This is a common-enough pattern, in my mind, that we should allow for it in the rbt API. So, this issue needs two things:

• a new ADR detailing what the new API should be for capturing stdout to a file
• after getting input on the ADR, implementing it

[celsobonutti]

I'd like to tackle this one

[BrianHicks]

PR for ADR is #91