Add simple vim mode

This commit adds a "vim_mode" setting (false/`0` by default) that causes
keys to be remapped in the following way by the `ScreenManager`:

+ h -> LEFT
+ j -> DOWN
+ k -> UP
+ l -> RIGHT
+ LEFT -> h (toggle help)
+ DOWN -> j (noop)
+ UP -> k (open kill menu)
+ RIGHT -> l (lsof current process)
+ K (Shift+K) -> k (open kill menu)
+ J (Shift+J) -> K (toggle show/hide kernel threads)
+ L (Shift+L) -> l (lsof current process)

I couldn't figure out where the manpage documentation is in the repo,
though I admittedly did not look particularly hard.

I believe this change would be a welcome option for heavy vim users like myself
who would like a familiar way to get around in htop.

ScreenManager.c

@@ -190,6 +190,23 @@ void ScreenManager_run(ScreenManager* this, Panel** lastFocus, int* lastKey) {
       set_escdelay(25);
       ch = getch();
 
+      if (this->settings->vimMode) {
+         switch (ch) {
+            case 'h': ch = KEY_LEFT; break;
+            case 'j': ch = KEY_DOWN; break;
+            case 'k': ch = KEY_UP; break;
+            case 'l': ch = KEY_RIGHT; break;
+            case KEY_LEFT: ch = 'h'; break;
+            case KEY_DOWN: ch = 'j'; break;
+            case KEY_UP: ch = 'k'; break;
+            case KEY_RIGHT: ch = 'l'; break;
+            case 'K': ch = 'k'; break;
+            case 'J': ch = 'K'; break;
+            case 'L': ch = 'l'; break;
+         }
+      }
+
+
       HandlerResult result = IGNORED;
       if (ch == KEY_MOUSE) {
          ch = ERR;