Wip/autoplugins

This is a work-in-progress pull-request. Feel free to add suggestions, corrections and ideas for the autoplugins. The current structure is

```
SbtNativePackager
    UniversalPlugin
        WindowsPlugin
        DockerPlugin
        LinuxPlugin
            RpmPlugin
            DebianPlugin
                DebianNativePackaging
                JDebPackaging
    AkkaAppPackaging
    JavaAppPackaging
        JavaServerAppPackaging
```

I will update the `Keys` here, when I have a final structure and everything works the first time.
Hmm, this was a big question in my head when I initially tried to do this when 0.13.5 was released.  Should we split the keys into different autoImports per-plugin or just have one glob?  Practically it probably doesn't matter, but you could see the sbt-native-packager was designed to have them be separate objects.