Allow result navigation w/ Tab and Shift+Tab

Closes #457

app/static/js/keyboard.js

@@ -1,8 +1,11 @@
 (function () {
     let searchBar, results;
+    let shift = false;
     const keymap = {
         ArrowUp: goUp,
         ArrowDown: goDown,
+        ShiftTab: goUp,
+        Tab: goDown,
         k: goUp,
         j: goDown,
         '/': focusSearch,
@@ -15,10 +18,21 @@
     });
 
     document.addEventListener('keydown', (e) => {
+        if (e.key === 'Shift') {
+            shift = true;
+        }
+
         if (e.target.tagName === 'INPUT') return true;
         if (typeof keymap[e.key] === 'function') {
             e.preventDefault();
-			keymap[e.key]();
+
+            keymap[`${shift && e.key == 'Tab' ? 'Shift' : ''}${e.key}`]();
+        }
+    });
+
+    document.addEventListener('keyup', (e) => {
+        if (e.key === 'Shift') {
+            shift = false;
         }
     });