Introduces two functions, PipePort and CapturePort, and implement output capture
in terms of them. These two functions return *Port instances, which can also be
used in (*Evaler).Eval calls.
As with commit #eb2a792 I acknowledge that `golint` recommendations are
controversial and should not automatically be acted on. Nonetheless,
this change fixes legitimate lint issues such as copy/paste cleanups,
style problems I introduced (`i += 1` versus `i++`) or method/var comments
that are not in the preferred form.
