This is a nice test (ha!) for how this works and what littlecheck can
do for us.
1. Input is now the actual file, not "Standard Input" anymore. So
any errors mentioning that now include the filename.
2. Regex are really nice for filenames, but especially for line
numbers
3. It's much nicer to have the output where it's created, instead of
needing to follow three files at the same time.
