Give `Cfgfifo` an "override" option

[jwodder]

Give Cfgfifo an option for setting a Format that "overrides" identify(), causing identify(), load(), and dump() to only return/operate on that format. This would be useful for applications that can take an explicit --format FORMAT command-line option to specify the format of a configuration file, disregarding its file extension.

Problem: What should this option be named? "override" is a reserved keyword in Rust, so while it could be used, it would look ugly.