Add processedWith(Iterable<? extends Processor>) to the subjects.

When testing Dagger, or other systems with more than one processor, it's convenient to do

```
public class MyTest {
  @Test public void aTest() { 
    ASSERT.that(javaSources())
        .that(testSource).processedWith(daggerProcessors()).hasNoErrors()
        .etc();
  }

  private final Iterable<? extends Processors> daggerProcessors() {
    return Arrays.asList(new Processor1(), new Processor2(), ...);
  }
}  
```

Without this, one must explicitly add each processor in each test, which is subject to failure-through-failed-duplication, etc.