add defaultAutoSession setting

When `config.defaultAutoSession` is false, generator will generate code such as `(implicit session: DBSession )` with unnecessary space after `DBSession`.

How about doing like this?

```scala
// add a space before '='
val defaultAutoSession = if (config.defaultAutoSession) s" = ${className}.autoSession" else ""
// remove a space in front of $defaultAutoSession
def save()(implicit session: DBSession$defaultAutoSession)
```