Add style plugins for appending additional styles to theme

[yhatt]

This PR will add style plugins for appending additional styles to theme. Marpit will parse <style> elements and its content as like as comment plugin, and parsed styles will append to a packing CSS. In pre-released Marp, using <style> elements is one of customization way of theme born from community.

The customized style within a markdown would be able to work with the same context of Marpit theme CSS.

Example

/* @theme example */
strong { color: red; }

---
theme: example
---

<style>
strong { color: blue; }
</style>

**The example**

In this case, the <strong> element will be colored blue.

Compiled CSS

/* @theme example */
div.marpit > section > strong { color: red; }
/* Append style */
div.marpit > section > strong { color: blue; }

The user specified style will append to the last of packed theme and be scoped automatically as same as a theme.

ToDo

• Add style elements plugin
• Append parsed style
• Support multiple <style> elements
• Support style global directive to keep compatibility with markdown for document
• Add test cases

[yhatt]

To avoid more parsing to already parsed tokens, I have changed the approach parsing <style> elements to a block rule as same as markdown-it's html parsing.

This approach will be able to apply #28 too.