composite script repositories

Right now, blowdryer only accepts a single, monolithic source of scripts and properties. But it would be nice to aggregate scripts from multiple places. One way to do that would be:

```gradle
blowdryerSetup {
forPrefixes ['scala/', 'clojure/'], {
// if a file (script, file, or prop) starts with `scala/` or `clojure/`
// then it will come from here
github 'diffplug/blowdryer-diffplug-experimental', 'tag', '5.7.0'
}

// at the bottom, you declare the fallback source
// for the case that it didn't match any prefixes
github 'diffplug/blowdryer-diffplug', 'tag', '2.0.0'
}
```