Fix slideshow stretching Fixes lathropd/reflowjs#6

Had to refactor a little because the negative margins on the image
container were messing with `max-width`
Fix alignment of vertical slideshow images

Not all of the vertical images were being center aligned because I had
reset the `figure`s to `max-width: 100%` instead of `width: 100%`, so
the images were centered in their containers but the containers weren’t
centered. Also fixed odd extra padding on first image - there’s
probably a reason it was set up that way though.
Minor - Code spacing cleanup
Fix implicit globals Fixes lathropd/reflowjs#14
Code whitespace cleanup
Fix bgImage scope issue

It broke because `img` is no longer global.
Fix hidden headline: Fixes lathropd/reflowjs#4
Merge pull request #12 from emilymerwin/master

Fix slideshow stretching Fixes lathropd/reflowjs#6