message: TabbarItem: fixed horizontal mode; readme updates
diff: max-width: 100%;
  padding: 0 2px;
  box-sizing: border-box;
  + align-items: center;
  + justify-content: center;
  }
  .Tabbar--l-vertical .TabbarItem__in {
  flex-direction: column;
  - align-items: center;
  - justify-content: center;
  }
  .Tabbar--l-horizontal .TabbarItem__in {
  flex-direction: row;
  - align-items: center;
  }
  .TabbarItem__icon {
  