Allow system plugins to provide publishing options

[elisee]

When clicking the Publish project button (http://docs.superpowers-html5.com/en/getting-started/publishing-your-game), rather than always downloading a ZIP archive of your exported game, a system and/or maybe one of its plugins should be able define different options.

Some use cases:

• Push a static website on GitHub Pages with Superpowers Web
• Upload a game or an update directly to itch.io with Superpowers Game or LÖVE
• Install or update a Minecraft resource pack and launch Minecraft with it
• etc.

[elisee]

As per the issue referenced above, we might want to make this not just about publishing options, but also provide different ways to generate the project when running it? Something to consider, at least.