fix: share icon size (#209)

Author: jizhang

source/css/_partial/article.styl

@@ -257,7 +257,7 @@ $article-share-link
   position: relative
   color: #999
   text-shadow: 0 1px #fff
-  &:before
+  .fa
     font-size: 20px
     absolute-center(@font-size)
     text-align: center