adding in center justify and left justify

@philtr I added in a justify left and center (cellucor needs it a ton, and it's kind of helpful). Please review the commit .for now, i also forked my own fondant-rails, for cellucor, so it points to my own version of fondant, until you guys merge this in.

It just adds a class of "center" or "left" to the element. It's up to the css/frontend dev to do anything with it.