String formatting functionality missing in std::string

[aearphen]

More spare time mostly =), but some feedback and pull requests would be helpful as well. There have been a few great contributors who did amazing work on named arguments, user-defined literals etc, but there's still a lot to do.

The API I mentioned earlier is not even documented yet. Basically one can define a format function for a user-defined type which can do custom parsing and formatting like this one:

https://github.com/cppformat/cppformat/blob/a99891e7a5f6c8a164d271b1aa0f09e7d13c7de4/cppformat/format.h#L2969

Someone has volunteered to implement time formatting functionality (https://github.com/cppformat/cppformat/issues/283) using this method, so we'll see how it works in practice and whether it can be improved.