Minimal change to permit non-interactive html diff generation (#468)

[cjerdonek]

This is a minimal PR to support addressing issue #468. Specifically, it contains a minimal code change needed to let html diffs be created non-interactively (e.g. from the command-line in CI) without having to patch nbdime's source code. A gist demonstrating this (in conjunction with this PR) can be found here: https://gist.github.com/cjerdonek/2dad48eea22846476487d594cd906493

This PR also doesn't even require any changes to the default template, though the gist I wrote above does support providing your own template if you want (which again doesn't require modifying nbdime's source code).

My hope is that, because this PR has a lighter footprint than PR #552, it can provide an interim and more flexible way to address issue #468 in the short-term so people no longer have to patch or fork nbdime to get the benefit of off-line html diffs.

[sizhky]

for those who can't wait for the merge and want this functionality, simply run

pip install git+https://github.com/cjerdonek/nbdime@issue-468-non-interactive-diffs

before trying out the gist provided

[cjerdonek]

Thanks, @sizhky. I have no idea why someone "thumbs-downed" this PR when there are no other simple work-arounds available. I've been using this PR with my team for two months now without issues (in conjunction with code similar to what's in the gist).