Support thousands separator

I know the docs say

> the ' printf option (format with thousands grouping characters)) has no effect in format.

and the code says
https://github.com/boostorg/format/blob/c1170a6d546b36f9399f3983fad0994e8f946d8f/include/boost/format/parsing.hpp#L172-L173

but it would be really neat if this could be supported in the future.

I assume the issue is that Boost::format is essentially trying to achieve all (or at least the majority) of its formatting using standard stream flags and there is none for thousands separators?

From https://stackoverflow.com/questions/17530408/print-integer-with-thousands-and-millions-separator it seems that what would be necessary is to use a [imbue](https://en.cppreference.com/w/cpp/io/ios_base/imbue) in combination with a locale.
Has this been considered yet?

EDIT: This approach seems to at least break when using Boost::multiprecision numbers with the stream at which case the thousands separator is ignored again.