Add --no-single-file-header flag

[niklasmohrin]

Closes #1311

Hey there! This was a pretty straight-forward change, let me know if I missed anything or if you want to have this handled differently :)

[sharkdp]

Thank you very much for your contribution!

@eth-p @keith-hall I'm not really convinced we should a new command-line option for this. Having a header for a single file is not really useful if someone is really just calling bat example.rs. But it's useful in a lot of other scenarios, e.g. when using bat to copy&paste code (or a screenshot of it) somewhere. So I don't want to disable the single file header by default.

I wonder if we could extend the --style option (again) to support this somehow?

[sharkdp]

I'd rather not implement this for now. Thank you, @niklasmohrin