standard output/error/input streams

[Phillip Lord]

I've worked up the patch that I made to add input streams for writing to
the system standard out. There was some discussion about this in Jul.

https://lists.gnu.org/archive/html/emacs-devel/2016-07/msg00910.html

The new version is on feature/stdout-stderr-stream.

Additions since last time. I've updated the names
(external-standard-output, external-standard-error) and I have also
added an input stream (external-standard-input).

The main motivation for this is the same as last time; it gives a
communication channel with Emacs which does not involve buffers at any
point. Although, if you search for "Emacs" and "standard output", other
people would like the same thing for other reasons.

Can I add this to master?

Phil