Add prop to style the date in 'infoTime' in eventMarker

Firstly, brilliant library - thank you.

As far as i can tell you cannot change the style on this element of the EventMarker.
It would be good if you could override this style as you can with `markerStyle` or `infoStyle` props.

It can sometimes be difficult to see the date on a busy chart.

I had a look at the code and added an extra prop - `infoTimeStyle` to override the default if present.
If this is something you would be interesting in adding let me know and i can submit a pull request.

Thanks