Add ssv.at method

[ryanve]

Get the value at the specified positive or negative index

ssv.at("mark tom travis", 0) // "mark"
ssv.at("mark tom travis", 1) // "tom"
ssv.at("mark tom travis", -1) // "travis"
ssv.at("mark tom travis", -2) // "tom"
ssv.at("mark tom travis", 5) // ""

[ryanve]

imma merge this to at least have it in ^3.8.0