Allow a DOM constructor function to be passed to placeholder

FEATURE: `placeholder` now allows a function that constructs the placedholder DOM to be passed
in, and uses `cloneNode` when a raw element is passed in, to prevent adding the same element
to multiple editors.

Closes codemirror/dev#1395

src/placeholder.ts

@@ -5,13 +5,16 @@ import {EditorView} from "./editorview"
 import {clientRectsFor, flattenRect} from "./dom"
 
 class Placeholder extends WidgetType {
-  constructor(readonly content: string | HTMLElement) { super() }
+  constructor(readonly content: string | HTMLElement | ((view: EditorView) => HTMLElement)) { super() }
 
-  toDOM() {
+  toDOM(view: EditorView) {
     let wrap = document.createElement("span")
     wrap.className = "cm-placeholder"
     wrap.style.pointerEvents = "none"
-    wrap.appendChild(typeof this.content == "string" ? document.createTextNode(this.content) : this.content)
+    wrap.appendChild(
+      typeof this.content == "string" ? document.createTextNode(this.content) :
+      typeof this.content == "function" ? this.content(view) :
+      this.content.cloneNode(true))
     if (typeof this.content == "string")
       wrap.setAttribute("aria-label", "placeholder " + this.content)
     else
@@ -35,7 +38,7 @@ class Placeholder extends WidgetType {
 
 /// Extension that enables a placeholder—a piece of example content
 /// to show when the editor is empty.
-export function placeholder(content: string | HTMLElement): Extension {
+export function placeholder(content: string | HTMLElement | ((view: EditorView) => HTMLElement)): Extension {
   return ViewPlugin.fromClass(class {
     placeholder: DecorationSet