SLF4D - A proposal for a common logging interface for Dub projects

[Andrew]

I've been spending some time in the last few weeks to prototype a 
logging framework that's inspired by 
[SLF4J](https://www.slf4j.org/). To that end, I've created 
[SLF4D](https://github.com/andrewlalis/slf4d), which provides a 
common logging interface, and a pluggable architecture to allow 
third-parties to handle log messages generated by any logger in 
an application.

Here's a short example of how it can be used in your code:

```d
import slf4d;

void main() {
   auto log = getLogger();
   log.info("This is an info message.");
   log.errorF!"This is an error message: %d"(42);
}
```

The library includes a default "logging provider" that just 
outputs formatted messages to stdout and stderr, but a 
third-party provider can be used by calling 
`configureLoggingProvider(provider)`.

The idea is that I can create logging providers to wrap the 
various logging facilities available in the D ecosystem already 
(Phobos, Vibe-D, etc.), so SLF4D can serve as a common interface 
to any provider.

I'd appreciate any feedback on this so far! This first version 
should be mostly stable, but there may of course be bugs. Thanks!