Adds nth operator (closes #51)

[Ezibenroc]

(the first element of the list is at position 1). If i < 0 then it returns the _i_th element from the end (nth(-1, l) returns the last element of the list).

Would be better to begin indexing at 0 (this is almost a standard in CS). Especially if -1 is the last element. Would be great to allow other negatives than -1.

[Tpt]

Would be better to begin indexing at 0

Yes, make sense

Would be great to allow other negatives than -1.

It's what is done: "If i < 0 then it returns the _i_th element from the end"

[Ezibenroc]

It's what is done: "If i < 0 then it returns the ith element from the end"

Sorry, read too fast.

Ok for me.

[rcharron]

Would be better to begin indexing at 0 (this is almost a standard in CS) => So be carefull when parsing, because in natural language, indexing start at 1

[Tpt]

Let's go with it!