Allow to customize flags for file decoders

[armanbilge]

E.g. to append to an existing file.

[rossabaker]

Some day I'll make up my mind how I feel about overloads, but I like the functionality and I don't have a better name for the new ones.

[armanbilge]

Yeah, I went through the same thought process on this one. At least it's not default args 🙃

[danicheg]

I like pure overloads but hate them powered with default arguments. There are none of them here, so that is greenlit from me 👍🏻

[rossabaker]

My only hesitation with this style of overload in general is that they are a nuisance for partial application. This breaks source compatibility. But I doubt very many people are using it that way here.