Sphinx html

Finally -- adds the ability to run Sphinx to the preview window.
I was thinking about it, but a bit hesitant: the user will see this info in the build output pane, so it seems redundant to put it here. But it is more convenient to get feedback here instead of after closing the settings dialog. I'm not sure what's best.

Another concern: if there are multiple Sphinx projects, each could (potentially) use a different sphinx-build (one for Py2, another for Py3, etc.), so there's not one check that covers everything.