Fixes https://github.com/sbt/sbt/issues/1502
This adds `--addPluginSbtFile=<file>` command, which adds the given .sbt file to the plugin build.
Using this mechanism editors or IDEs can start a build with required plugin.
```
$ cat /tmp/extra.sbt
addSbtPlugin("com.eed3si9n" % "sbt-assembly" % "0.14.7")
$ sbt --addPluginSbtFile=/tmp/extra.sbt
...
sbt:helloworld> plugins
In file:/xxxx/hellotest/
...
sbtassembly.AssemblyPlugin: enabled in root
```
Eugene Yokota