[FEATURE] remove infer-owner

[nlf]

What / Why

We would like this module to be a Promise-y wrapper around child_process.spawn and little else. The current features of buffering stdout/stderr streams and returning the final result (optionally as a string) make sense in this context, however the behavior of inferring a uid/gid to run a process as is outside of the scope of concerns of this module.

[Foorack]

As this magic uid/gid inferring was re-introduced, can this issue be re-opened?

[yuki-js]

seemed reverted in #12, but README is not reverted.