Improve and refactor examples (#1930)

This just refactors the examples to make the styled defined inline with each example, to make it easier to follow for folks. And in the process fixes a few issues that people brought up.

Fixes https://github.com/ianstormtaylor/slate/issues/1920
Fixes https://github.com/ianstormtaylor/slate/issues/1925

examples/plain-text/index.js

@@ -22,6 +22,22 @@ class PlainText extends React.Component {
     ),
   }
 
+  /**
+   * Render the editor.
+   *
+   * @return {Component} component
+   */
+
+  render() {
+    return (
+      <Editor
+        placeholder="Enter some plain text..."
+        value={this.state.value}
+        onChange={this.onChange}
+      />
+    )
+  }
+
   /**
    * On change.
    *
@@ -31,24 +47,6 @@ class PlainText extends React.Component {
   onChange = ({ value }) => {
     this.setState({ value })
   }
-
-  /**
-   * Render the editor.
-   *
-   * @return {Component} component
-   */
-
-  render() {
-    return (
-      <div className="editor">
-        <Editor
-          placeholder="Enter some plain text..."
-          value={this.state.value}
-          onChange={this.onChange}
-        />
-      </div>
-    )
-  }
 }
 
 /**