Add support for *nix shell style file varargs

[vitling]

There is currently no way to pass through "main parameter" varargs, as might be expanded from a shell wildcard like "*.txt".

Possible mechanisms include:

• A magic-named parameter which implies it'll take all the remaining args and parse them into a Seq
• Allowing arbitrary varags by the possibility of multiple values for each --
• Specialisation of the last arg at the type level by special-casing Seq[T] :: HNil
• Annotation to suggest that a field should accept the "everything else" varargs.