don't spawn an editor if stdin & stdout aren't a terminal

I sometimes run `ent show ISSUE | more` because ISSUE doesn't fit on my
screen and we don't have built-in pager support yet.

Then i see something i want to change, or i want to add a comment, so
i up-arrow in the shell and change `show` to `edit` or `comment`, and
ent opens what is in effect `vi | more` which isn't what i want at all.