New feature: Plugin management & update API

That would be awesome if plugins management and updating would be automated.

Here's my view on that:

There is already _File menu -> Settings window -> Plugins tab_, where right now there is just plugins folder setting. What about if there would be also some kind of _Add plugin -> GitHub full project name (group/name)_ field, and EDMC would fetch from GitHub API latest release, download zip archive, extract it in plugins directory, and restart EDMC.

Updating scenario: fetch from GH latest release, compare versions ([via packaging?](https://stackoverflow.com/a/11887885)), download new release if needed, clean old plugin directory and extract new version.

Also, there could be Update API specification for projects that aren't hosted on GihHub. In this case, in plugins tab there would be _Add plugin -> Custom URL_, that should return next JSON:
```
{
"latest_version": "1.4.0",
"zip_url": "..."
}
```

I can try to implement that by myself when I'll have some spare time.