Implement option for custom output destinations in output.Get()

[mbestavros]

Currently, output.Get() always defaults to creating a new Outputter object that writes to os.Stdout; however, it's possible that a different output channel is desired.

This PR adds an option to allow a different File object as output when calling Get().

Resolves #879.

[boranx]

That won't have an effect on how the cli outputs the result, will it?
as far as I can imagine, that might be useful when conftest is used as a lib
If so, could you open up an issue with the rationale for the change for tracking purposes?

[mbestavros]

That won't have an effect on how the cli outputs the result, will it?

@boranx It shouldn't. Just allowing an option to redirect that output somewhere else.

as far as I can imagine, that might be useful when conftest is used as a lib

That's why I proposed it! 😄

If so, could you open up an issue with the rationale for the change for tracking purposes?

Done!

[boranx]

lgtm

Thanks for the contribution