Consider adding `StringSubject.containsAll([String...|Iterable<String>])`

I bumped into this yesterday on a work project - I was personally surprised to see that these aren't valid assertions in Truth:

```java
assertThat("One ring to rule them all").containsAll("One", "all", "rule");
assertThat("One ring to rule them all").containsAll(ImmutableSet.of("to", "them", "One"));
```

I actually wrote a `StringSubject2` class with `containsAll(Iterable)` method and associated tests yesterday to cover this need.

Am I missing something obvious in `StringSubject` or elsewhere in Truth that would meet my need?

If not, would this be something the Truth team would be willing to consider, and would the team be happy to accept a PR with the code I developed to kick the discussion off?