v1.7.4 - Update shortcut getter

What's new

1. Update method graph.IsShortcut.
   Old:

// IsShortcut Returns true if edge is a shortcut (edge defined as two vertices)
func (graph *Graph) IsShortcut(labelFromVertex, labelToVertex int64) (ok bool) {
	_, ok = graph.shortcuts[labelFromVertex][labelToVertex]
	return ok
}

New - Now maps proper source/target IDs + returns ViaVertex ID:

// IsShortcut Returns (vertex_id; true) if edge is a shortcut (edge defined as two vertices)
//
// If source or taget vertex is not found then returns (-1; false)
// If edge is not a shortcut then returns (-1; false)
//
func (graph *Graph) IsShortcut(labelFromVertex, labelToVertex int64) (int64, bool) {
	source, ok := graph.mapping[labelFromVertex]
	if !ok {
		return -1, ok
	}
	target, ok := graph.mapping[labelToVertex]
	if !ok {
		return -1, ok
	}
	shortcut, ok := graph.shortcuts[source][target]
	if !ok {
		return -1, ok
	}
	return graph.Vertices[shortcut.ViaVertex].Label, ok
}