Legend supports opacity channel

[YuhanLu]

Fix #1349
Now legend supports to show opacity channel

{
  "data": {"url": "data/seattle-weather.csv", "formatType": "csv"},
  "mark": "bar",
  "config": {
    "mark": { "opacity": 0.5 }
  },
  "encoding": {
    "x": {
      "field": "date",
      "type": "temporal",
      "timeUnit": "month"
    },
    "y": {
      "field": "*",
      "type": "quantitative",
      "aggregate": "count"
    },
    "opacity": {
      "field": "*",
      "type": "quantitative",
      "aggregate": "count"
    }
  }
}

[kanitw]

// Avoid override default mapping for opacity channel

[YuhanLu]

Do you have any suggestion how should we avoid this? Since symbols are initialized in applyMarkConfig() (line 157), I think it's better to do any modification in legend.ts instead of other places.

[kanitw]

I think this is fine. I mean add a comment

// Avoid override default mapping for opacity channel

Sorry for not being clear.

[kanitw]

Still missing test and I guess we need to wait for a new Vega release, which is coming soon?