Simple site plugins

[HelloZeroNet]

Have ability to modify the site layout/add new features without modifying the original source code to keep it updateable.

Eg if you create a plugins/DarkSkin/all.css file on all.css request it would also append this file's content to the end. (same for js)

Maybe combining something similar like hotpatching in #322, so it would be possible to easily attach new functions to events.

[obv-mikhail]

How would this be different from adding new features in like this css/plugins/DarkSkin/all.css ?

Would the above compile (append to css/all.css) currently?

[obv-mikhail]

@HelloZeroNet Maybe add support for multiple databases per site? This way a plugin can store its data independently from the rest of the site. Would make adding some features much easier.

[sergei-bondarenko]

@HelloZeroNet Have this implemented in 0.5.5 and can be closed? I am talking about "Upgrade code" feature.