Extend textBetween API by accepting function to stringify leaf nodes

FEATURE: `textBetween` now allows its leaf text argument to be a function.

src/fragment.js

@@ -40,7 +40,7 @@ export class Fragment {
     this.nodesBetween(0, this.size, f)
   }
 
-  // :: (number, number, ?string, ?string) → string
+  // :: (number, number, ?string, ?string | ?(leafNode: Node) -> string) → string
   // Extract the text between `from` and `to`. See the same method on
   // [`Node`](#model.Node.textBetween).
   textBetween(from, to, blockSeparator, leafText) {
@@ -50,7 +50,7 @@ export class Fragment {
         text += node.text.slice(Math.max(from, pos) - pos, to - pos)
         separated = !blockSeparator
       } else if (node.isLeaf && leafText) {
-        text += leafText
+        text += typeof leafText === 'function' ? leafText(node): leafText
         separated = !blockSeparator
       } else if (!separated && node.isBlock) {
         text += blockSeparator

src/node.js

@@ -93,7 +93,7 @@ export class Node {
   // children.
   get textContent() { return this.textBetween(0, this.content.size, "") }
 
-  // :: (number, number, ?string, ?string) → string
+  // :: (number, number, ?string, ?string | ?(leafNode: Node) -> string) → string
   // Get all text between positions `from` and `to`. When
   // `blockSeparator` is given, it will be inserted whenever a new
   // block node is started. When `leafText` is given, it'll be

test/test-node.js

@@ -86,6 +86,20 @@ describe("Node", () => {
                "paragraph", "foo", "bar", "image", "baz", "hard_break", "quux", "xyz"))
   })
 
+  describe("textBetween", () => {
+    it("works when passing a custom function as leafText", () => {
+      const d = doc(p("foo", img, br))
+      ist(d.textBetween(0, d.content.size, '', (node) => {
+        if (node.type.name === 'image') {
+          return '<image>'
+        }
+        if (node.type.name === 'hard_break') {
+          return '<break>'
+        }
+      }), 'foo<image><break>')
+    })
+  })
+
   describe("textContent", () => {
     it("works on a whole doc", () => {
       ist(doc(p("foo")).textContent, "foo")