Trailing commas in map and list literals

[eevee]

This is being discussed on the Ruby tracker and I think we should totally do it. Allow them in lists and maps, and allow a single comma to make a one-element list. See also: nex3/sass#917

$list-of-three-items: (
    1,
    2,
    3,
);

$one-by-three-list: (1 2 3),;

$map: (
    key: value,
);