Multiple tooltips with different trigger type

### What problem does this feature solve?
For my line chart I need to show two types of tooltips. One on hover on exact point(like with trigger item) and another on hovering any other place (like trigger axis).
Below issue was raised for same but closed without proper response
https://github.com/apache/incubator-echarts/issues/4905

### What does the proposed API look like?
```js
tooltip: [{
trigger: 'axis',
formatter: axisTooltipFormatter
},{
trigger: 'item',
formatter: itemTooltipFormatter
}]
```
Or
```js
{
tooltip: {
trigger: 'axis',
formatter: axisTooltipFormatter
},
series[{
...
tooltip: {
formatter: itemTooltipFormatter // item trigger
}
]
```