::backdrop selector support for inline SVG mode

[yhatt]

Added Marpit PostCSS SVG backdrop plugin.

Retarget ::backdrop and section::backdrop selector to @media screen { :marpit-container > svg[data-marpit-svg] { .. } }. It means ::backdrop targets the SVG container in inline SVG mode.

It's useful for setting style of the letterbox and pillarbox in the SVG scaled slide. (marp-team/marp-cli#358, marp-team/marp#183)

::backdrop {
  background-color: #448;
}

The original definition will remain to support an original usage of ::backdrop.

The important differences from an original ::backdrop are following:

• In original spec, ::backdrop creates a separated layer from the target element, but Marpit's ::backdrop does not. The slide elements still become the child of ::backdrop so setting some properties that are inherited may make broken slide rendering.
• Even if the browser is not fullscreen, ::backdrop will match to SVG container whenever matched to @media screen media query.

For developers: If concerned to conflict with the style provided by the integrated app, consider to disable the selector support by inlineSVG: { backdrop: false }.

Close #313.

ToDo

• inlineSVG option object
• Tests
• Update type definition