feature request - support icu date format string directly

We have a case where we receive date/time formatting strings in icu format - but are using `strftime`/`std::chrono` style format strings in the codebase.

As I believe Boost::Locale converts `strftime` strings to ICU format when using the ICU backend, it would be useful to be able to pass the ICU format string through to the backend directly.

This is something I'd be happy to work on myself if it would likely be accepted.

Of course we could use ICU directly but as you've observed the API is not all that friendly.