Extend API to add/remove unknown plugins

[RolandJansen]

Followup to #116

[RolandJansen]

This is tricky (at this point of development).

1. We need to setup a project with a reference to intermix >=0.5
2. Intermix 0.5 has not been released and the API isn't stable
3. We could use a manual build of intermix but since the API is still changing, this can get really painful

So this is something that should be done right after the release of 0.5.

[RolandJansen]

There are two new API function

• addPluginClass(PluginClass: IPluginConstructor): void
• removePluginClass(className: string): boolean

Plugins are simply ES2015 modules that can be dynamically loaded into the browser like this:

import * as intermix from "<path-to-file>";  // just to clarify where the "intermix" object comes from
[...]
import("<url>").then((PluginModule) => {
    intermix.addPluginClass(PluginModule.default);
}

or even simpler

const PluginModule = await import("<url>");
intermix.addPluginClass(PluginModule.default);

see d592c1cde990d7591a4ca9168a1c10fb8995dbe5