Add anchor links to heading (#38)

* Closes #36

assets/scss/style.scss

@@ -22,6 +22,7 @@ html {
   background: $light-grey;
   line-height: 1.6;
   letter-spacing: .06em;
+  scroll-behavior: smooth;
 }
 
 body,
@@ -534,6 +535,39 @@ hr.post-end {
     padding: 0;
     margin-left: 1.8em;
   }
+
+  a.anchor {
+    float: left;
+    margin-left: -20px;
+    padding-right: 6px;
+    box-shadow: none;
+    opacity: .8;
+    &:hover {
+      background: none;
+      color: $theme;
+      opacity: 1;
+    }
+
+    svg {
+      display: inline-block;
+      width: 14px;
+      height: 14px;
+      vertical-align: baseline;
+      visibility: hidden;
+    }
+    &:focus svg {
+      visibility: visible;
+    }
+  }
+
+  h1:hover a.anchor svg,
+  h2:hover a.anchor svg,
+  h3:hover a.anchor svg,
+  h4:hover a.anchor svg,
+  h5:hover a.anchor svg,
+  h6:hover a.anchor svg {
+    visibility: visible;
+  }
 }
 
 .footnotes {