added alignat latex directive support

Seems like some equations, like the one in http://www.feynmanlectures.caltech.edu/I_09.html which comes after "free fall" word sequence, are scoped by /begin{alignat} and /end{alignat} which aren't currently supported. I didn't change to a more general regex that would capture /begin{.*} because it seemed to me that you probably had reasons against it (as it is simpler and would eliminate the need for both /begin{equation} and /begin{align} expressions) , if there aren't any, I think a more general regex would be better.

Thanks :)