Make InMemorySink disposable to clear log events

[sandermvanvliet]

This PR addresses issue #3 to allow disposing the logger to clear out all the log events that were previously sent to the sink.

Depending on your test framework and test setup this feature may come in handy when you want to reset the log events.

Consider for example MSTest:

public class WhenDemonstratingDisposableFeature
{
    private Logger _logger;

    [TestInitialize]
    public void Initialize()
    {
        _logger?.Dispose();

        _logger = new LoggerConfiguration()
            .WriteTo.InMemory()
            .CreateLogger();
    }

    [TestMethod]
    public void GivenAFoo_BarIsBlah()
    {
        _logger.Information("Foo");

        InMemorySink.Instance
            .Should()
            .HaveMessage("Foo");
    }

    [TestMethod]
    public void GivenABar_BazIsQuux()
    {
        _logger.Information("Bar");

        InMemorySink.Instance
            .Should()
            .HaveMessage("Bar");
    }
}

this approach ensures that the GivenABar_BazIsQuux does not see any messages logged in a previous test.