func Append(to interface{}, from ...interface{}) (interface{}, error)
Append appends from to a slice to and returns the resulting slice.
If length of from is one and the only element is a slice of same type as to,
it will be appended.
type Slicer interface {
Slice(items interface{}) (interface{}, error)
}
Slicer defines a very generic way to create a typed slice. This is used
in collections.Slice template func to get types such as Pages, PageGroups etc.
instead of the less useful []interface{}.