Added support for rel='noopener'

Author: vkallore

Diff for: docs/configuration.md

@@ -405,6 +405,19 @@ window.$docsify = {
 };
 ```
 
+## externalLinkRel
+
+- type: `String`
+- default: `noopener`
+
+Default `'noopener'` (no opener) prevents the newly opened external page (when [externalLinkTarget](#externallinktarget) is `'_blank'`) from having the ability to control our page. No `rel` is set when its not `'_blank'`.
+
+```js
+window.$docsify = {
+  externalLinkTarget: '' // default: 'noopener'
+};
+```
+
 ## routerMode
 
 - type: `String`

Diff for: src/core/config.js

@@ -24,6 +24,7 @@ export default function () {
       mergeNavbar: false,
       formatUpdated: '',
       externalLinkTarget: '_blank',
+      externalLinkRel: 'noopener',
       routerMode: 'hash',
       noCompileLinks: [],
       relativePath: false

Diff for: src/core/render/compiler.js

@@ -78,6 +78,7 @@ export class Compiler {
     this.toc = []
     this.cacheTOC = {}
     this.linkTarget = config.externalLinkTarget || '_blank'
+    this.linkRel = this.linkTarget === '_blank' ? (config.externalLinkRel || 'noopener') : ''
     this.contentBase = router.getBasePath()
 
     const renderer = this._initRenderer()
@@ -185,7 +186,7 @@ export class Compiler {
 
   _initRenderer() {
     const renderer = new marked.Renderer()
-    const {linkTarget, router, contentBase} = this
+    const {linkTarget, linkRel, router, contentBase} = this
     const _self = this
     const origin = {}
 
@@ -243,6 +244,7 @@ export class Compiler {
         href = router.toURL(href, null, router.getCurrentPath())
       } else {
         attrs += href.indexOf('mailto:') === 0 ? '' : ` target="${linkTarget}"`
+        attrs += href.indexOf('mailto:') === 0 ? '' : (linkRel !== '' ? ` rel="${linkRel}"` : '')
       }
 
       if (config.target) {