fix: add template functions can return HTMLElement (#836)

In my own use case, I'm trying to attach a Vue component containing an image. Some properties are computed against `scrollHeight` of this image, and this is 0 if the element is not attached to the DOM.

So returning `outerHTML` is not the same thing as returning the element itself.

types/index.d.ts

@@ -227,7 +227,7 @@ export type TimelineOptionsMarginType = number | TimelineMarginOption;
 export type TimelineOptionsOrientationType = string | TimelineOrientationOption;
 export type TimelineOptionsSnapFunction = (date: Date, scale: string, step: number) => Date | number;
 export type TimelineOptionsSnapType = null | TimelineOptionsSnapFunction;
-export type TimelineOptionsTemplateFunction = (item?: any, element?: any, data?: any) => string;
+export type TimelineOptionsTemplateFunction = (item?: any, element?: any, data?: any) => string | HTMLElement;
 export type TimelineOptionsComparisonFunction = (a: any, b: any) => number;
 export type TimelineOptionsGroupHeightModeType = 'auto' | 'fixed' | 'fitItems';
 export type TimelineOptionsClusterCriteriaFunction = (firstItem: TimelineItem, secondItem: TimelineItem) => boolean;