std.logger

[Volcz]

I would to drop some requirements I've seen in the Java world.

* As klamonte mention in the pull request on GitHub: Appenders 
are a must in a enterprise enviorment.
* Same with filters. In SLF4J normally each class has it's own 
logger which can be filtered or appended to individual files.
* Runtime configuration. Crucial when collection data about bugs 
in running systems.
* Performance... The possibility to write asynchronous loggers.
* Sometimes it's expensive to construct the log output. In Java 
for example Logback calls toString on objects after it has 
concluded that the data is needed. So a possibility to pass an 
function for constructing the log output only when the logger is 
sure that it's going to be logged.

My 2 SEK. ;-)