Allow disabling of Validation#flatMap deprecation warning

Like so.

```scala
scala> import scalaz._, syntax.validation._
import scalaz._
import syntax.validation._

scala> 1.success flatMap (x => (x + 3).failNel)
<console>:20: warning: method ValidationFlatMapDeprecated in object Validation is deprecated: flatMap does not accumulate errors, use `scalaz.\/` or `import Validation.FlatMap._` instead
              1.success flatMap (x => (x + 3).failNel)
                ^
res1: scalaz.Validation[scalaz.NonEmptyList[Int],Nothing] = Failure(NonEmptyList(4))

scala> import Validation.FlatMap._
import Validation.FlatMap._

scala> 1.success flatMap (x => (x + 3).failNel)
res2: scalaz.Validation[scalaz.NonEmptyList[Int],Nothing] = Failure(NonEmptyList(4))
```