Added clean-tex and TEXFLAGS

clean-tex removes all tex related build files

added TEXFLAG variable for all XeLaTeX commandline arguments
muted output of XeLaTeX (redirected stdout)
set XeLaTeX to halt on errors.
Fixed answer key generation and detection.

To enable answer key generation, yaml file must have variable answerkey
Added graphicx package to XeLaTeX template.

graphicx enables \resizebox and \scalebox for modifying tables.
Added prototype for quiz 2 on memory.