Add support for IConfiguration when using netstandard.

[niemyjski]

This allows for other consumers using .NET Core clients like WPF to use IConfiguration.

ExceptionlessClient.Default.Configuration.ReadFromConfiguration(config);
ExceptionlessClient.Default.Register();

We are taking a dep on the LTS abstraction, it would be great to automatically read from configuration but:

1. We would have to take out much larger dependencies..
2. We wouldn't know your naming scheme of your configuration files...