Returns the element at index index in the list stored at key.
The index is zero-based, so 0 means the first element, 1 the second element
and so on.
Negative indices can be used to designate elements starting at the tail of the
list.
Here, -1 means the last element, -2 means the penultimate and so forth.
When the value at key is not a list, an error is returned.
@return
@bulk-string-reply: the requested element, or nil when index is out of range.
@examples
LPUSH mylist "World"
LPUSH mylist "Hello"
LINDEX mylist 0
LINDEX mylist -1
LINDEX mylist 3