Please publish an Automatic-Module-Name

Hello.

When using `auto` to generate code for a modular Java 9 project, you'll get complaints from tools about depending on an automatically-generated module name.

Is there any chance you could add an `Automatic-Module-Name` entry in the jar manifest in order to provide a stable name upon which other Java 9 projects can depend? I'd suggest:

```
Automatic-Module-Name: com.google.auto.value
```