fix(cover): Compatible with legacy styles, fixed #677 (#678)

Please makes sure these boxes are checked before submitting your PR, thank you!

* [ ] Make sure you are merging your commits to `master` branch.
* [ ] Add some descriptions and refer relative issues for you PR.
* [ ] DO NOT include files inside `lib` directory.

Author: QingWei-Li

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

@@ -265,8 +265,6 @@ export class Compiler {
         result = helperTpl('tip', text)
       } else if (/^\?>/.test(text)) {
         result = helperTpl('warn', text)
-      } else if (/^</.test(text)) {
-        return text
       } else {
         result = `<p>${text}</p>`
       }

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

@@ -179,7 +179,7 @@ export function renderMixin(proto) {
 
     const m = html
       .trim()
-      .match('<img.*?data-origin="(.*?)"[^a]+alt="(.*?)">([^<]*?)$')
+      .match('<p><img.*?data-origin="(.*?)"[^a]+alt="(.*?)">([^<]*?)</p>$')
 
     if (m) {
       if (m[2] === 'color') {
@@ -198,17 +198,6 @@ export function renderMixin(proto) {
       html = html.replace(m[0], '')
     }
 
-    // XXX: A Workaround
-    html = html
-      .split('\n')
-      .map(part => {
-        if (/^<a/.test(part)) {
-          return part.replace('<a', '<a data-button')
-        }
-        return part
-      })
-      .join('\n')
-
     this._renderTo('.cover-main', html)
     sticky()
   }

Diff for: src/themes/basic/_coverpage.styl

@@ -61,7 +61,7 @@ section.cover
     max-width 500px
     padding 0
 
-  .cover-main > a[data-button]
+  .cover-main > p:last-child a
     border-color var(--theme-color, $color-primary)
     border-radius 2rem
     border-style solid