Consider add `--dry-run` option to `fix` command

[Dentrax]

It would be nice to have such --dry-run flag to see what protolint going to rewrite:

For example:

$ protolint lint -fix . --dry-run

[yoheimuta]

@Dentrax Thank you for bringing up a nice suggestion.

I agree with you, but it could take some time to add this option because the current implementation is not well-designed for it.