[Feature] legend.formatter

### What problem does this feature solve?

```
legend.formatter(name)         ---------->  legend.formatter(name,serieName)  or    legend.formatter(name,serieOption) 

```

### What does the proposed API look like?

 ```
 legend.formatter(name,serieName)  
```

 or    
```
series：{

  legendFormatter(name){
  }
}
```