Issue #3: Refactoring ILogger

Moved extension method `IsEnabled` to the `ILogger` interface, renamed `WriteCore` to `Write` and changed the return type to void. This will break other repositories that use the ILogger interface, which I will update once I get feedback on the changes here.
Not part of your PR, but the regular `<` check in a for loop is easier to grok 
`for (var index = 0; index < _loggers.Length; index++)`