Add support for `NonBlankString` and `StrippedString`

Java 11 introduces new methods in `String`: `isBlank` and `strip`. Those methods complement good-old `isEmpty` and `trim` respectively but sometimes are way better suitable for processing locale-specific strings.

It should be fairly easy to capture this functionality in "refined" types like `NonBlankString` and `StrippedString` (that would complement `NonEmptyString` and `TrimmedString` respectively).