Add warning / force option when overwriting files

[DennisGaida]

As the documentation says to use sops encrypt file > file.enc it is the first thing I tried, but didn't like that it just overwrites files without asking before doing so. I thought the --output option would alleviate that and warn me when writing to an existing file - nope. This is actually how I got to checking out the CLI documentation with a small word error: #1975

Anyways, I would love if the --output option would not just overwrite a file without asking. This may be any file anywhere so I believe it is quite a dangerous operation. I would love if we had a -y/--yes option or a --force option that would need to be added to not have any warnings. This goes for sops encrypt --output <file> as well as for sops decrypt --output <file>.

I know this would be a breaking change, but it frightens me a bit using sops (just like rm -rf is always frightening, but it already has the f for fearnot) without any check for existing files.