Feature: Support cloning of es6 Maps

[omerts]

Currently when trying to clone an ES2015 Map object, I get an empty object returned.

[pvorb]

What platform are you using? Which browser/Node.js version? Browserify, Webpack, etc.?

[tristanls]

I'm not sure if this is the correct place to put this, but lack of support for Maps breaks clone in newest Node.js.

$ node -v
v6.5.0
$ node
> const clone = require("clone");
undefined
> const map = new Map();
undefined
> map.set("key", "value");
Map { 'key' => 'value' }
> map.has("key");
true
> const clonedMap = clone(map)
undefined
> clonedMap.has("key");
TypeError: Method Map.prototype.has called on incompatible receiver #<Map>
    at Map.has (native)
    at repl:1:11
    at sigintHandlersWrap (vm.js:22:35)
    at sigintHandlersWrap (vm.js:96:12)
    at ContextifyScript.Script.runInThisContext (vm.js:21:12)
    at REPLServer.defaultEval (repl.js:313:29)
    at bound (domain.js:280:14)
    at REPLServer.runBound [as eval] (domain.js:293:12)
    at REPLServer.<anonymous> (repl.js:504:10)
    at emitOne (events.js:101:20)