Add resolvers options to plugin API

```
var transpiledLib = compileModules(lib, {
  formatter: 'bundle',
  resolvers: [ NPMResolver ],
  output: 'app.js'
});
```
Do you think it will be clear that they're passing a constructor and not an instance? `options.resolverClasses` might be more clear, but won't look as nice :) I'm on the fence… I guess either way we'll document this feature.