configuration for making other tasks conditional to checksums changing

Currently to make a task conditional based on checksums changing, it requires some build setup like the following:

```
publish.dependsOn computeChecksums
publish.onlyIf {
   ! computeChecksums.sameAsPropertyFile()
}
publish.finalizedBy saveChecksums
```

It would be nice if the `checksum` configuration extension offered a way in which this could be done more easily.
