add details / collapsable / accordion markup

[danlangford]

id like to propose a new markup tag that can collapse away a bunch of text. some frameworks call these "accordions". we could put this around long explanations, large spoilers, or giant quotes.

[details]a lot of text[/details]

[details=A Summary Or Title Here]a lot of text, probably tons of statistics that bagels likes to put in the forums[/details]

here is how it could look and work. (click the ▸ below)

a lot of text

A Summary Or Title Here

a lot of text, probably tons of statistics that bagels likes to put in the forums

and fortunately all modern browsers already support this functionality that requires no CSS or JS

<details>a lot of text</details>

<details>
    <summary>A Summary Or Title Here</summary>
    a lot of text, probably tons of statistics that bagels likes to put in the forums
</details>

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do arguments to a markup tag support spaces? if not, and this a summary/title being passed in is problematic then I might suggest alternative syntax

[details]a lot of text[/details]

[details=open]a lot of text, probably tons of statistics that bagels likes to put in the forums. this one will default to being open[/details]

which could work like this

a lot of text

a lot of text, probably tons of statistics that bagels likes to put in the forums. this one will default to being open

and would have a syntax like this

<details>a lot of text</details>

<details open>
a lot of text, probably tons of statistics that bagels likes to put in the forums. this one will default to being open
</details>