Prefix templating

```javascript
var logger = require('fastlog')('configured', 'error', '${level} [${ category }] <${timestamp}>');
logger.error('This town ain\'t big enough for the two of us!');
// error [configured] <Fri, 05 Dec 2014 02:10:48 GMT> This town ain't big enough for the two of us!
```

cc @willwhite