fix: allow axis settings when ("bin":"binned").

[orbisvicis]

Before, any axis blocks would be dropped from the resulting Vega, including any
axis settings. After, mirror the behavior of ("bin":true) - axis settings cause
the axis block to be included.

This because I wanted grid lines to accompany my pre-binned data.

Test case:

{ "mark": "bar",
  "encoding": {
    "x": {
      "type": "temporal",
      "bin": "binned",
      "field": "start",
      "axis": {"grid": true}
    },
    "x2": {"field": "end"},
    "y": {"type": "quantitative", "field": "value"}
  },
  "transform": [
    {"calculate": "toDate(datum.day)", "as": "day"},
    {"calculate": "timeOffset('hours', datum.day, -12)", "as": "start"},
    {"calculate": "timeOffset('hours', datum.day, 12)", "as": "end"}
  ],
  "height": 250,
  "width": 800,
  "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
  "data": {
    "values": [
      {"day": "2021-01-01T00:00:00", "value": 5},
      {"day": "2021-01-02T00:00:00", "value": 4},
      {"day": "2021-01-04T00:00:00", "value": 5},
      {"day": "2021-01-05T00:00:00", "value": 4},
      {"day": "2021-01-09T00:00:00", "value": 1},
      {"day": "2021-01-10T00:00:00", "value": 3},
      {"day": "2021-01-11T00:00:00", "value": 2},
      {"day": "2021-01-12T00:00:00", "value": 3},
      {"day": "2021-01-13T00:00:00", "value": 5},
      {"day": "2021-01-15T00:00:00", "value": 3}
    ]
  }}

Please:

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Tips:

• https://medium.com/@greenberg/writing-pull-requests-your-coworkers-might-enjoy-reading-9d0307e93da3 is a nice article about writing a nice PR.
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[domoritz]

Thank you for the pull request. This looks great.

Some axis settings such as "axis": {"tickColor": "hotpink"} worked beforehand as well.