use (...args) {
    this.plugins.push(...args)

    return this
  }
@this posthtml
@param   {Function} plugin - A PostHTML plugin
@returns {Constructor} - this(PostHTML)

**Usage**
```js
ph.use((tree) => { tag: 'div', content: tree })
  .process('<html>..</html>', {})
  .then((result) => result))
```