add converter for shapeless singleton literals

[yangzai]

Not sure why you might need this but this PR allows you to constrain config fields to singleton literal values.

Example with Scala 2.13 (you will need Witness for Scala 2.12):

case class Profile(name: "foo", age: 42, theAnswerIsAlways: "traverse")
ConfigSource.string("{ name = foo, age = 42, the-answer-is-always: traverse }").load[Profile]

Technically you could also do the same with refined-pureconfig (e.g. Int Refined Equal[42]) but it's not as neat as with singleton literals.