Style figure captions a bit

[mcatanzaro]

This helps with #72 but it's not a complete fix, since the content may not be aligned to the left-hand side of the page.

[ignatenkobrain]

@mcatanzaro probably we should add something like width: 100% to avoid issues with aligning?

[mcatanzaro]

width: 100% seems to make it worse for me

I am trying to find something better. It doesn't help that I never learned CSS. :(