This is an helper method that will first try to find the item with the
desired name using getItem(). If the item is found, it will call
its setContent() method. If not, it will just create
a new directive at the bottom with the given name and content.
Parameter
string
$name
Name of the directive to look for
mixed
$content
New content, usually a string
integer
$index
Index of the directive to set, in case there are more than one directive with the same name