Shorthand singleton marks

For single-instance marks, it’d be neat if you could just pass options without data. For example:

Plot.text({
  text: "Hello, world!"
  x: new Date("2016-05-01"),
  y: 100,
  dx: -10,
  dy: 40,
  frameAnchor: "top-left"
})

Could be equivalent to:

Plot.text({length: 1}, {
  text: ["Hello, world!"],
  x: [new Date("2016-05-01")],
  y: [100],
  dx: -10,
  dy: 40,
  frameAnchor: "top-left"
})

It feels like the helper would need to know which options possibly represent channels, though. And there would potentially be ambiguity with alternative representations of data (such as Arquero objects #449); though that could be addressed by checking the number of arguments and whether the first is a plain object (Object.getPrototypeOf === Object.prototype).

Start at the Plot.text entry point and inspect how its arguments are currently interpreted. Compare the proposed one-object form with the existing two-argument form, including plain-object and alternative data representations. Done means singleton marks accept the shorthand without making existing calls ambiguous, with coverage for the new and existing forms.