Support multiple FindBugs files

Support multiple "before" and "after" files. They will be read together and their `instanceHash`'s merged together into the internal map for diffing.

Perhaps implement as an additive CLI options:

```
--before "pre-1.xml" --before "pre-2.xml" --after "post-1.xml" --after "post-2.xml"
```

and/or

```
--before "pre-1.xml" "pre-2.xml" --after "post-1.xml" "post-2.xml"
```

http://java.dzone.com/articles/groovy-multiple-values-single

Also think about wildcard support, e.g. using `AntBuilder`'s `fileScanner`.