Add toggle for auto map-centering

public/index.html

@@ -117,8 +117,18 @@
         </label>
       </div>
 
-      <div id="toggle-grid-vis">
-        <input type="checkbox" checked />
+      <div class="opt-toggle">
+        <input type="checkbox" id="toggle-grid-vis" checked />
+        <label for="toggle-grid-vis">
+          <span class="off">⬡</span><span class="on">⬢</span>
+        </label>
+      </div>
+
+      <div class="opt-toggle">
+        <input type="checkbox" id="auto-center-map" />
+        <label for="auto-center-map">
+          <span class="off">⦾</span><span class="on">⦿</span>
+        </label>
       </div>
 
       <div id="toggle-firing-arc-vis">