google-java-format Eclipse Plugin

Enabling

Uncomment <module>eclipse_plugin</module> in the parent pom.xml
Run mvn install, which will copy the dependences of the plugin to eclipse_plugin/lib.
If you are using Java 9, add
```
-vm
/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/bin/java
```
to /Applications/Eclipse.app/Contents/Eclipse/eclipse.ini.
Open the eclipse_plugin project in a recent Eclipse SDK build.
From File > Export select Plugin-in Development > Deployable plugin-ins and fragments and follow the wizard to export a plugin jar.