[Feature] Print name of file before displaying its contents

Running shellharden or multiple files is confusing, since there's no clear delimiter between their contents. Current behavior is akin to cat, while a bat-like mode would be more appropriate.

[anordal]

I think this is a good idea. Maybe line numbers too. I'm less sure about that, but it relates in terms of what should be on by default.

Then comes the question of which options to add, to enable/disable these features.