Add \input command support

[mwest1066]

From Dallas:

I think it could be helpful to have support for the \input command. I realize there are work-arounds to this, but when I write my exams, I put the individual questions in their own tex files, then put a series of input commands into the TeX file. It makes moving and reusing questions easy, and you can also see just from the tex file what coverage you have on topics. I know running a quick preprocessor could do this too, before passing it to rand-exam; it's an extra step.