Overwrite files options

[heijul]

When asking, whether a file should be overwritten, adding "overwrite [a]ll/never [o]verwrite" could make sense.
At the same time, this part was originally written, when the output consisted of different .txt files instead of a proper feed. In case this function is only needed for the output feed, there is no need to change it (though we should add a comment explaining the reasoning).