Support keyword `horizontal` for remaining plots

[randyzwitch]

Area and boxplot specifically.

[randyzwitch]

Area:

{
  "width": 400,
  "height": 400,
  "padding": {"top": 10, "left": 30, "bottom": 30, "right": 10},
  "data": [
    {
      "name": "table",
      "values": [
        {"x": 1,  "y": 28}, {"x": 2,  "y": 55},
        {"x": 3,  "y": 43}, {"x": 4,  "y": 91},
        {"x": 5,  "y": 81}, {"x": 6,  "y": 53},
        {"x": 7,  "y": 19}, {"x": 8,  "y": 87},
        {"x": 9,  "y": 52}, {"x": 10, "y": 48},
        {"x": 11, "y": 24}, {"x": 12, "y": 49},
        {"x": 13, "y": 87}, {"x": 14, "y": 66},
        {"x": 15, "y": 17}, {"x": 16, "y": 27},
        {"x": 17, "y": 68}, {"x": 18, "y": 16},
        {"x": 19, "y": 49}, {"x": 20, "y": 15}
      ]
    }
  ],
  "scales": [
    {
      "name": "x",
      "type": "linear",
      "range": "width",
      "nice": true,
      "domain": {"data": "table", "field": "y"}
    },
    {
      "name": "y",
      "type": "linear",
      "range": "height",
      "zero": false,
      "domain": {"data": "table", "field": "x"}
    }
  ],
  "axes": [
    {"type": "x", "scale": "x", "ticks": 20},
    {"type": "y", "scale": "y"}
  ],
  "marks": [
    {
      "type": "area",
      "from": {"data": "table"},
      "properties": {
        "enter": {
          "interpolate": {"value": "monotone"},
          "orient": {"value": "horizontal"},
          "y": {"scale": "y", "field": "x"},
          "x2": {"scale": "x", "field": "y"},
          "x": {"scale": "x", "value": 0},
          "fill": {"value": "steelblue"}
        },
        "update": {
          "fillOpacity": {"value": 1}
        },
        "hover": {
          "fillOpacity": {"value": 0.5}
        }
      }
    }
  ]
}

[randyzwitch]

Area works, but stacked area doesn't. In general, coord_flip!() probably needs a re-write