This adds underscored initializers that let library users add `= nil` to
declarations of optional `@Option` and `@Argument` properties. Previously,
default values have been available for properties of non-optional types
only.
These new initializers use `_OptionalNilComparisonType` as the wrapped
value parameter, so only a `nil` literal is acceptable in the default
value position. This avoids the problem of declaring an optional property
with a non-`nil` default, which ends up negating the purpose of an optional.
Nate Cook
KthandNate Cook