Add option for a custom formatter

[znxuz]

I figured it'd be nice to let the user to have the opt-in for choosing the vsnprintf implementation they see fit by adding a macro surrounding the single formatter call to vsnprintf.

The vsnprintf from stdio.h seems to be not reentrant/malloc-free even when configUSE_NEWLIB_REENTRANT is set in the config and causes hard fault in my benchmark code originally for another freertos lib, so that i couldn't even do the benchmark against the vsnprintf impl. from for example printf.h to present the results here.

But maybe with this addition the ulog could also claim that it would be reentrant when both the formatter and the logger for the formatted result are reentrant?