Support -1 in slice notation

[sparkprime]

Essentiually, finish off #106

[davidzchen]

To recap from the offline discussion, @oconnorr brought up the point that having negative indices often results in error-prone code and that it would be better to not support them. I think that is a valid point and am also in favor in not adding negative indices.

[sparkprime]

Yep, let's leave them unsupported as we can simply do

local x = arr; x[length(x) - 1]

or

(function(x)x[length(x) - 1])(arr)

to represent

arr[-1]

Although it's more chars, it's not often that you need to use negative indexes whereas index arithmetic errors can happen anywhere.