Add intro image overlay SCSS variable

To disable intro image color accent overlay add set the following SCSS variable to `false` like so:

$intro-image-color-overlay: false;

Fixes #55

README.md

@@ -603,6 +603,12 @@ author:
 comments: false  # disable comments on this post
 ```
 
+**Note:** Hero images are overlaid with a transparent "accent" color to unify them with the theme's palette. To disable [customize the CSS](#customizing-sass-scss) with the following Sass variable override:
+
+```scss
+$intro-image-color-overlay: false;
+```
+
 ### `layout: page`
 
 Visually this layout looks and acts the same as `layout: post`, with two minor 

_sass/basically-basic/_intro.scss

@@ -19,7 +19,9 @@
     right: 0;
     bottom: 0;
     left: 0;
+    @if $intro-image-color-overlay == true {
       background-color: rgba($accent-color, 0.25);
+    };
     content: '';
   }
 

_sass/basically-basic/_variables.scss

@@ -84,6 +84,9 @@ $sidebar-width: $small !default;
 /* Background color of the sidebar */
 $sidebar-background-color: #fff !default;
 
+/* Intro image color overlay */
+$intro-image-color-overlay: true !default;
+
 /* Border radius */
 $border-radius: 0.25em !default;