Image floating support like in MDWiki

## Feature request

#### What problem does this feature solve?

Nice image placement that only needs simple markdown convention.

#### What does the proposed API look like?

Basically in .md files, place image elements with different line breaks with text and renderer can render the images in different ways.

More details can be found in http://dynalon.github.io/mdwiki/#!layout.md#Image_floating.

#### How should this be implemented in your opinion?

(With a feature switch to keep compatibility), we can generate elements of images with layout options.

#### Are you willing to work on this yourself?

Not quite familiar with HTML etc.

未指定任何仓库文件、测试或入口点。先阅读提议的 Markdown 换行约定和链接的 MDWiki 图片浮动参考；在确定实现和测试之前，明确兼容性开关和渲染行为。

由索引模型根据 Issue 内容生成。