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.
Based on my comment about when `IsEnabled` should be `true`/`false` we should add a test that has more than one logger registered and try all 4 `Boolean` combinations of enabled/disabled for each of two registered loggers.

BTW to do this you can use the `[Theory]` feature of xUnit.net. Check out other tests in other repos to see how you can do this with `InlineData` or I can give you a hand.