Bring back XML for configuration files

[asvishnyakov]

I want to use XML (not this stupid JSON!). Please, allow to use alternative language (XML perfect) for configuration files.

[davidfowl]

We do allow xml configuration files. Just not web.config. We support JSON, XML and INI files.

[Eilon]

Already supported. See:

1. Blog post from Louis: http://whereslou.com/2014/05/23/asp-net-vnext-moving-parts-iconfiguration/
2. Unit tests: https://github.com/aspnet/Configuration/blob/dev/test/Microsoft.Framework.ConfigurationModel.Xml.Test/XmlConfigurationSourceTest.cs