ENH: handle trace attributes similar to x and y in convenience api

See the inconsistency here:

```
julia> print(json(plot([1 2; 3 4], marker_color=[:red :blue]), 2))
{
"layout": {
"margin": {
"l": 50,
"b": 50,
"r": 50,
"t": 60
}
},
"data": [
{
"marker": {
"color": [
[
"red"
],
[
"blue"
]
]
},
"y": [
1,
3
],
"type": "scatter",
"x": [
1,
2
]
},
{
"marker": {
"color": [
[
"red"
],
[
"blue"
]
]
},
"y": [
2,
4
],
"type": "scatter",
"x": [
1,
2
]
}
]
}
```