Show a permalink anchor when hovering over headings in main content (#2251)

* Implement heading permalinks, close #2246

Thanks to jekyll/jekyll for CSS.
Link anchor is visible when the mouse hovers over the title line.

* Build the updated _main.js

_sass/minimal-mistakes/_page.scss

@@ -72,6 +72,23 @@
     border-bottom: 1px solid $border-color;
   }
 
+	h1, h2, h3, h4, h5, h6 {
+		.header-link {
+			position: relative;
+			left: 0.5em;
+			opacity: 0;
+			font-size: 0.8em;
+			-webkit-transition: opacity 0.2s ease-in-out 0.1s;
+			-moz-transition: opacity 0.2s ease-in-out 0.1s;
+			-o-transition: opacity 0.2s ease-in-out 0.1s;
+			transition: opacity 0.2s ease-in-out 0.1s;
+		}
+
+		&:hover .header-link {
+			opacity: 1;
+		}
+	}
+
   p,
   li,
   dl {