Add sweep plugin

[yhatt]

Marpit sweep plugin will hide blank paragraph tokens. Normally the background image syntax ![bg]() would be written as paragraph of Markdown. The following example is the result of rendering without a background syntax support.

![bg](https://example.com/bg.jpg)

<p><img src="https://example.com/bg.jpg" alt="bg" /></p>

The same case would find in Marpit directives. Marpit can parse HTML comment without html: true markdown option, so it would be wrapped by paragraph when markdown-it has disabled html option.

<!-- theme: default -->

<p>&lt;!-- theme: default --&gt;</p>

When the multiple HTML comments or background images in a paragraph are rendered, the whitespace and breaks between tokens would keep as the content of paragraph.

![bg](https://example.com/bg1.jpg)
![bg](https://example.com/bg2.jpg)

<p>  <- softbreak between images
</p>

We don't expect to be rendered the paragraph with the blank content. Marpit sweep plugin provides a common logic to hide the paragraph and its content.

ToDo

• Add test of Marpit sweep plugin
• Remove the logic removing blank paragraph from Marpit comment plugin
• Update background image plugin to be able to sweep
  • Split and add image parser plugin

EDIT: Background image plugin that is implemented in #5 can not apply sweep logic because of context of the order of plugin. We updated background image plugin to be better.