split should include capture groups in results

[pythe]

As a web developer approaching Elm from Javascript, I expect regexes and associated functions to behave the same as in JS.

Issue: In Elm 0.18 Core/Regex, an expression containing capture groups would splice captures into the returned array. In Elm 0.19 Regex, the capture groups are omitted.

Justification: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/split#Description

If separator is a regular expression that contains capturing parentheses, then each time separator is matched, the results (including any undefined results) of the capturing parentheses are spliced into the output array.

Examples:

---- Elm 0.19.0 ----------------------------------------------------------------
Read <https://elm-lang.org/0.19.0/repl> to learn more: exit, help, imports, etc.
--------------------------------------------------------------------------------
> import Regex
> Regex.split (Maybe.withDefault Regex.never <| Regex.fromString ",") "a,b,c,d"
["a","b","c","d"] : List String
> Regex.split (Maybe.withDefault Regex.never <| Regex.fromString "(,)") "a,b,c,d"
["a","b","c","d"] : List String

---- elm-repl 0.18.0 -----------------------------------------------------------
 :help for help, :exit to exit, more at <https://github.com/elm-lang/elm-repl>
--------------------------------------------------------------------------------
> import Regex exposing (regex, HowMany(All))
> Regex.split All (regex ",") "a,b,c,d"
["a","b","c","d"] : List String
> Regex.split All (regex "(,)") "a,b,c,d"
["a",",","b",",","c",",","d"] : List String

// This was run in Chrome 69
console.log("a,b,c,d,e".split(/,/));
console.log("a,b,c,d,e".split(/(,)/));
VM59:1 (5) ["a", "b", "c", "d", "e"]
VM59:2 (9) ["a", ",", "b", ",", "c", ",", "d", ",", "e"]

[pythe]

I'm willing to try providing a pull request if this issue is valid.