Ability to specify max choices supported by scantron or whatever

[armandofox]

Our scantrons only allow choices A-E per answer, but you can specify additional choices by combining multiple letters, eg AB, AC, etc. HTML and other output modes of Ruql should support a 'max_choices' option so that, eg, if it is set to 3, then a 5-choice question would print the choices as A, B, C, AB,AC (or similar), and an error would be raised if any questions are "select all that apply". Of course, given the finding that 3 choices are optimal for most MCQs, this is low priority :-)