Add --keep-synced-files to the 'apply' command

[ineu]

Per https://github.com/sorah/hocho/issues/13.

This options is only applicable to ssh-based drivers, i.e.: bundler, mitamae. It's not applicable to itamae_ssh as this driver doesn't copy the files, so there's nothing to keep or remove.

Drivers are set to ignore unknown arguments passed via kwargs.

[ineu]

@sorah just in case you've missed the PR :)