Adding support for protoc plugins

Hi there,

I added support for referencing protoc plugins from the build script. A script using this feature looks like this:

```groovy
java {
	plugin = ['plugin 1.exe', 'plugin 2.exe']
}
```

As plugins are also platform specific, it may be better to have platform specific paths in the same way as for protoc itself. But if you only use one platform, this approach works quite well.

Regards,
Jakob Wenzel