Add option to add null separator to blocks command

[gdavison]

To help using terrafmt blocks in toolchains, add the option to use a null separator. This would allow e.g. piping to xargs -0 ...

There doesn't seem to be consistency between existing tools:

• find uses -print0
• GNU grep uses -Z, --null
• sort uses -z, --zero-terminated

I propose using --zero-terminated for the long option and -z for the short option, or possibly -0.