Fixed irregular code block borders (#1260)

* fix irregular radius on code samples

* fixed inconsistent radii on selected code blocks

Author: mychidarko

src/.vuepress/theme/styles/code.styl

@@ -31,6 +31,7 @@ div[class*="language-"]
   background-color #f6f6f6
   border-radius 6px
   .highlight-lines
+    border-radius 6px
     background-color #f6f6f6
     color #3a385d
     user-select none