Best way to model optional arguments

This is a FAQ on the Scala community, especially made by newcomers.

For example:

• Here in SO: Implicit conversion from A to Some(a)
• On the users' Discourse: https://users.scala-lang.org/t/passing-true-optional-arguments-to-functions/6087
• In the Gitter Scala channel: https://gitter.im/scala/scala?at=5d28f90d01621760bca2eae3