flot: fixed type for gridOptions.borderWidth

According to [the API document](https://github.com/flot/flot/blob/master/API.md), gridOptions.borderWidth can be a number or an object.

> "borderWidth" is the width of the border around the plot. Set it to
> 0 to disable the border. Set it to an object with "top", "right",
> "bottom" and "left" properties to use different widths.

This pull request changes gridOptions.borderWidth to type any. 
And also this property is modified as optional property.