Support replacing invalid utf-8 with U+FFFD

[djmitche]

In support of GothenburgBitFactory/taskwarrior#3333 -- older Taskwarrior's create JSON by just blasting bytes into a file, and if that contains invalid utf-8 then the import process will not work.

@lauft I see you've made other changes in this repo. Is this something you could work on, by any chance?

[djmitche]

@lauft I recall having a look at how this was implemented in libshared and it wasn't clear how to fix it. Is it something you are familiar with?