Allow user to specify queries in validation sampling

[stsievert]

It'd be nice if the user could specify the target indices a user asks about. i.e.,

targets: ["a", "b", "c", "d", "e"]
samplers:
  ValidationSampling: {queries: [(0, 1, 2), (1, 2, 3), (2, 3, 4)]}

would show these targets to the user:

• ["a", "b", "c"] (via target indices 0, 1, 2)
• ["b", "c", "d"] (via target indices 1, 2, 3)
• ["c", "d", "e"] (via target indices 2, 3, 4)

cc @kushinm