ui.cli: Don't record EOF or blank entries as answers

If the user closes stdin, don't check and see if 'EOF' is an allowed
answer, just exit the QuestionCommandLine loop.  Don't try to process
blank entries either, just re-ask the question.
question: Add the ChoiceQuestion.multiple_answer attribute

This adds support for multiple choice questions where you are allowed
to select one or more of the possible choices.  In web forms, this is
usually done via checkboxes.
quizzes/software-carpentry-pre-assessment: Allow multiple platforms

For mixed-OS environments (e.g. local OS X machine with department
Linux boxes).