Support CSS scoping by element id

[yhatt]

This PR will support CSS scoping by element id.

On ThemeSet#pack, Marpit's container and slideContainer have been used to genearte a scoped CSS selector. It supports scoping by class attribute, but sometimes you might want higher specificity.

We will improve PostCSS pseudo selector replace plugin to recognize id attribute.

const container = new Element('div', { id: 'container' })
const { css } = new Marpit({ container }).render('# Markdown')

/* Internal pseudo selector */
:marpit-container > :marpit-slide { background: #fff; }

/* After convert */
div#container > section { background: #fff; }

This feature might use in @marp-team/marp-cli's bespoke template.