Add defaultValue prop to Editor component (#2418)

* Add `defaultValue` prop to `Editor` component

* Use `defaultValue` prop in the `Read Only` example

* Use `defaultValue` prop in the `Check Lists` example

* Use `defaultValue` prop in the `Code Highlighting` example

* Use `defaultValue` prop in the `Embeds` example

* Use `defaultValue` prop in the `Emojis` example

* Use `defaultValue` prop in the `Forced Layout` example

* Use `defaultValue` prop in the `Huge Document` example

* Use `defaultValue` prop in the `Images` example

* Use `defaultValue` prop in the `Input Tester` example

* Use `defaultValue` prop in the `Markdown Preview` example

* Use `defaultValue` prop in the `Markdown Shortcuts` example

* Use `defaultValue` prop in the `Paste HTML` example

* Use `defaultValue` prop in the `Plain Text` example

* Use `defaultValue` prop in the `Plugins` example

* Use `defaultValue` prop in the `RTL` example

* Use `defaultValue` prop in the `Search Highlighting` example

* Use `defaultValue` prop in the `Tables` example

examples/plain-text/index.js

@@ -3,6 +3,16 @@ import { Editor } from 'slate-react'
 
 import React from 'react'
 
+/**
+ * Deserialize the initial editor value.
+ *
+ * @type {Object}
+ */
+
+const initialValue = Plain.deserialize(
+  'This is editable plain text, just like a <textarea>!'
+)
+
 /**
  * The plain text example.
  *
@@ -10,18 +20,6 @@ import React from 'react'
  */
 
 class PlainText extends React.Component {
-  /**
-   * Deserialize the initial editor value.
-   *
-   * @type {Object}
-   */
-
-  state = {
-    value: Plain.deserialize(
-      'This is editable plain text, just like a <textarea>!'
-    ),
-  }
-
   /**
    * Render the editor.
    *
@@ -32,21 +30,10 @@ class PlainText extends React.Component {
     return (
       <Editor
         placeholder="Enter some plain text..."
-        value={this.state.value}
-        onChange={this.onChange}
+        defaultValue={initialValue}
       />
     )
   }
-
-  /**
-   * On change.
-   *
-   * @param {Editor} editor
-   */
-
-  onChange = ({ value }) => {
-    this.setState({ value })
-  }
 }
 
 /**