Make `outputs` not stack content

[bennyhat]

We've been supporting a weird use case for outputs where multiple configurations using it append together rather than override each other.

Ex.

stubbed_command.outputs('hello')
stubbed_command.outputs('world')

This outputs 'helloworld' to stdout, which is not very intuitive to most users. Ideally, that should just output 'world'.

We'll have to bump our minor version for this, as it's a significant change to behavior