Playground support

[plinss]

It would be useful to have a 'playground' to experiment with Bikeshed syntax without having to have (or output) a full spec. i.e a mode where it doesn't require a metadata block, skips all the boilerplate and just outputs the entered text. I'd find this useful when making small edits to other's specs, such as when editing a PR.

I'd be happy to add support to https://api.csswg.org/bikeshed/, maybe this can be something like a pre-canned metadata block that the site can prepend to the input when in 'playground' mode.