Do not show header if path is a directory

```
> bat some-directory
[bat error]: Is a directory (os error 21)
───────┬──────────────────────────────────────────────────
       │ File: some-directory
───────┼──────────────────────────────────────────────────

```

When calling `bat` on a directory (more typically, when using `bat *`), we should not print a header for this entry.