Implement auto generate slides from playground

[hiroppy]

Type

• bug
• feature request

For Feature Request

We can avoid writing MarkDown as much as possible if fusuma can create slides from playground which is on browsers.

MarkDown and CSS will be generated automatically from UI but I don't want to destroy the MarkDown code.

MD syntax

<!-- id: hash -->
# hello

#hash {}

So... all parts have an id selector.