Adding a new preference

Adding a new preference

Edit PreferenceDescriptor.scala to add new preference descriptor:

case object PlaceScaladocAsterisksBeneathSecondAsterisk extends BooleanPreferenceDescriptor {
  val key = "placeScaladocAsterisksBeneathSecondAsterisk"
  val description = "Place Scaladoc asterisks beneath the second asterisk in the opening '/**', as opposed to the first"
  val defaultValue = false
}

Also add to AllPreferences list in same file.