cli: Add --select to filter the stack by index

For example, you can show the questions in a quiz with either the
`config` tag or the `checkout` tag:

  $ ./pq.py --all --tag config --tag checkout --questions quizzes/git.json
  Question 0:
  Configure your user-wide name to be `A U Thor`.

  Question 1:
  Configure your user-wide email to be `author@example.com`.

  Question 2:
  You've messed up your README file.
  Restore it to the last committed version.

Say questions 0 and 2 look interesting, and you'd like to try question
2 first.  Run:

  $ ./pq.py --all --tag config --tag checkout -s 2 -s 0 quizzes/git.json