watermark is too simple

[Mister-Hope]

The watermark can be easily removed, at least the feature should use a MutationObserver to enforce a watermark once the original one is being removed.

I am suggesting watermark-js-plus for this feature, it's not heavy and powerful enough for this, and as mentioned in the other issue, it should be the first option for a plugin like @vuepress/plugin-watermark

[pengzhanbo]

提交了一个 PR #111 , 使用 watermark-js-plus 为 VuePress 站点添加水印。 主题后续将会切换到这个新插件。

[Mister-Hope]

The pr looks great, but need a little tweaks.