[v1.6 beta] add header-mask support, finally!

Author: Huxpro

_layouts/post.html

@@ -9,10 +9,21 @@
 <!-- Post Header -->
 <style type="text/css">
     header.intro-header{
+        position: relative;
         background-image: url('{{ site.baseurl }}/{% if page.header-img %}{{ page.header-img }}{% else %}{{ site.header-img }}{% endif %}')
     }
+
+    {% if page.header-mask %}
+    header.intro-header .header-mask{
+        width: 100%;
+        height: 100%;
+        position: absolute;
+        background: rgba(0,0,0, {{ page.header-mask }});
+    }
+    {% endif %}
 </style>
 <header class="intro-header" >
+    <div class="header-mask"></div>
     <div class="container">
         <div class="row">
             <div class="col-lg-8 col-lg-offset-2 col-md-10 col-md-offset-1">