Enable instantiating logger with format-string

[manne65-hd]

Hi there @ FatFreeFramework,
let me first say THANK YOU for such a great piece of software!

I am currently working on a "big" cronjob, that uses a lot of logging. As I prefer to use a different format string for date/time, it kind of annoyed me, to add my custom format-string to every write-method-call.

So I gave it a shot and extended the log-class with a feature to instantiate it with a desired format-string that will be used for all subsequent write-method-calls that don't pass a format-string.

As far as I have tested, this retains full backward compatibilty, while just adding the new feature.

If you find it useful, maybe you can merge it.

Thanks again for FatFreeFramework,
Manfred