Add warning about using linkify option

lib/index.js

@@ -56,6 +56,15 @@ function Remarkable(preset, options) {
     preset = 'default';
   }
 
+  if (options && options.linkify != null) {
+    console.warn(
+      'linkify option is removed. Use linkify plugin instead:\n\n' +
+      'import Remarkable from \'remarkable\';\n' +
+      'import linkify from \'remarkable/linkify\';\n' +
+      'new Remarkable().use(linkify)\n'
+    );
+  }
+
   this.inline   = new ParserInline();
   this.block    = new ParserBlock();
   this.core     = new ParserCore();

test/linkify.js

@@ -1,4 +1,5 @@
 import path from 'path';
+import assert from 'assert';
 import { addTests } from './utils';
 import Remarkable from '../lib/index';
 import linkify from '../lib/linkify';
@@ -7,3 +8,19 @@ describe('linkify plugin', function () {
   var md = new Remarkable({ html: true }).use(linkify);
   addTests(path.join(__dirname, 'fixtures/linkify.txt'), md);
 });
+
+describe('linkify option', function () {
+  it('should warn about using linkify option instead of plugin', () => {
+    const messages = []
+    const oldWarn = console.warn;
+    console.warn = message => messages.push(message);
+    var md = new Remarkable({ html: true, linkify: true });
+    console.warn = oldWarn;
+    assert.deepEqual(messages, [
+      `linkify option is removed. Use linkify plugin instead:\n\n` +
+      `import Remarkable from 'remarkable';\n` +
+      `import linkify from 'remarkable/linkify';\n` +
+      `new Remarkable().use(linkify)\n`
+    ])
+  });
+});