Deal with pandoc center-aligning images

[cjerdonek]

Running pandoc on our Markdown files adds--

:align: center

to images, whereas we want images to be left-aligned. We should have a programmatic way to deal with this (i.e. instead of manually deleting the alignment directive). Also see this thread.

[cjerdonek]

@jgm fixed this issue very shortly after it was reported (see the thread linked to above), so this issue should be resolved now.

[cjerdonek]

I confirmed that :align: center is no longer added when running python setup.py prep in commit eb3fc2d.