style: optimize style

[imfenghuang]

1. use flex layout to replace the float layout;

flex is beautiful and it has also been used in other places of this plugin

2. move the li hover animation to the li:hover::before

If there is a hover animation on li element and the position of li element will move up and down during the animation, causing the hover style of li element switch back and forth (from cursor: pointer; to cursor: auto; then to cursor: pointer;) when the mouse is on the edge of li element.
Move the animation from li to the li::before, the problem has been resolved. And the li cursor: pointer; is also still working normally.