/dev/zero fix (final)

[sharifhsn]

Fixes #200.

Instead of flushing on every single print, it flushes only when the first line is printed. This produces the expected behavior for hexyl /dev/zero:

❯ target/debug/hexyl /dev/zero
┌────────┬─────────────────────────┬─────────────────────────┬────────┬────────┐
│00000000│ 00 00 00 00 00 00 00 00 ┊ 00 00 00 00 00 00 00 00 │⋄⋄⋄⋄⋄⋄⋄⋄┊⋄⋄⋄⋄⋄⋄⋄⋄│
^C

[sharkdp]

Thank you!