Allow variadic arguments to m.fragment (#2328)

* Implement support for variadic arguments to `m.fragment`

While I was at it, I refactored the common logic out of `hyperscript`.

* Add a missed change from #2326

* Update docs + changelog [skip ci]

* Explain rationale for `hyperscriptVnode`'s calling convention

This way, it doesn't get erroneously "cleaned up" into something worse,
and so it's clearer how it'd be potentially optimized once ES5 support
is dropped.

render/fragment.js

@@ -1,7 +1,12 @@
 "use strict"
 
 var Vnode = require("../render/vnode")
+var hyperscriptVnode = require("./hyperscriptVnode")
 
-module.exports = function(attrs, children) {
-	return Vnode("[", attrs.key, attrs, Vnode.normalizeChildren(children), undefined, undefined)
+module.exports = function() {
+	var vnode = hyperscriptVnode.apply(0, arguments)
+
+	vnode.tag = "["
+	vnode.children = Vnode.normalizeChildren(vnode.children)
+	return vnode
 }