Add optional custom format specifiers to DDLogMessageFormat about cocoalumberjack

[ffried]

Unfortunately it's not that easy. With your proposed solution, the following code would be accepted by the compiler:

DDLogDebug("Some float value: \(someFloat, specifier: "whatever")")

Which would then lead to a runtime crash.

The proper way would be to add fully typed format specifiers, similar to what OSLog does.

In the meantime, I'd recommend using FormatStyle if possible. This will lead to the parameter being recorded as string, though. However, since we're not (yet) doing much with the concrete values (except for recording the format), that's fine for now.