introduce readList<T> function

- Read a bracket-delimited list, or handle a single value as list of size 1.
  Mostly useful for handling command-line arguments.
  eg,
      if (args.options().found("patches"))
      {
          patches = readList<word>(IStringStream(args.options()["patches"])());
      }
  can handle both of these:
      -patches  patch0
      -patches \( patch1 patch2 patch3 \)