embedded REPLs

[Rich-Harris]

Adds a /repl/embed page, and uses it to render a few interactive examples on the homepage:

TODO:

• The transition example needs some more work (not least because transitions are currently broken in v3)
• the layout is a bit rubbish when the screen is narrow
• use demo= instead of gist=, so the examples are versioned

[Conduitry]

I understand if you don't want to deal with it for now, but I think we want to try to use the demo= mechanism instead of using Gists for this.

[Rich-Harris]

Yeah, I guess you're right. I've updated the TODO list in the top comment

[Conduitry]

I just updated the demos to use the integrated easing and transition functions. The embedded transition example's custom-transitions.js should also use them.

[Rich-Harris]

working on it as we speak — also moving over to the demo= thing

[Rich-Harris]

guess i should have squashed this branch huh