servant-js: export CommonGeneratorOptions constructors
servant-js: export CommonGeneratorOptions constructors

Allow defCommonGeneratorOptions to be customized using record update
syntax.

Without the re-export, we can't do:

```
import qualified Servant.JS as SJS

myOptions =
  SJS.defCommonGeneratorOptions
    { SJS.moduleName = "myModule"
    , SJS.urlPrefix = "http://localhost"
    }
```

zerobuzz/servant,zerobuzz/servant