Possibility to set specific timezone for time axis

### What problem does this feature solve?
In our case, we have many clients arround the world. We want have the ability to show their data using their timezone. Actually, Echarts use GMT dates to set xAxis intervals labels and current browser timezone for date format.

We correctly set the correct timezone in formatter but, for intervals, we have already the problem of GMT.

It's a really important feature for us.

### What does the proposed API look like?
An option, in xAxis to set timezone should be great to fix the problem :

```
xAxis : {
type : 'time',
timezone : 'Europe/Paris', // with that, all xAxis times are manipulated using this timestamp
}
```