Add well-known actual, expected fields

Closes GH-12.

Reviewed-by: Titus Wormer <tituswormer@gmail.com>

index.js

@@ -111,6 +111,18 @@ export class VFileMessage extends Error {
     // Feel free to add other non-standard fields to your messages.
 
     /* eslint-disable no-unused-expressions */
+    /**
+     * You can use this to specify the source value that’s being reported, which
+     * is deemed incorrect.
+     * @type {string?}
+     */
+    this.actual
+    /**
+     * You can use this to suggest values that should be used instead of
+     * `actual`, one or more values that are deemed as acceptable.
+     * @type {Array<string>?}
+     */
+    this.expected
     /**
      * You may add a file property with a path of a file (used throughout the VFile ecosystem).
      * @type {string?}

readme.md

@@ -129,6 +129,16 @@ Stack of message (`string?`).
 It’s OK to store custom data directly on the `VFileMessage`, some of those are
 handled by [utilities][util].
 
+###### `actual`
+
+You can use this to specify the source value that’s being reported, which is
+deemed incorrect.
+
+###### `expected`
+
+You can use this to suggest values that should be used instead of `actual`, one
+or more values that are deemed as acceptable.
+
 ###### `file`
 
 You may add a `file` property with a path of a file (used throughout the