Specifying language prevents detection of file existance.

[EmilGedda]

As topic, should be easy to fix.

[EmilGedda]

https://github.com/EmilGedda/hattis/blob/master/app/Main.hs#L141
(lang input) should use allfiles from Hattis.Text.SourceFile in some way to verify the existance of the files.