Configuration API changes (absolute file paths no longer supported)

[HaoK]

For more details see: aspnet/Configuration#407

Overview of changes:

• IConfigurationSource has been introduced to represent the settings/configuration which is used to Build() a IConfigurationProvider. It is no longer possible to access the provider instances from IConfigurationBuilder only the sources. This is intentional, but may cause loss of functionality as you can longer do things like explicitly call Load() on the provider instances.
• FileConfigurationProvider base class has been introduced as a common root for Json/Xml/Ini providers. This allows the ability to specify an IFileProvider on the source which will be used to read the file instead of explicitly using File.Open. The side effect of this change is that absolute paths are no longer supported, the file path must be relative to the base path of the IConfigurationBuilder's basepath or the IFileProvider if specified.
• Example of configuring sources:

new ConfigurationBuilder()
      .SetBasePath(@"C:\SomeOherDirectory")
      .AddIni("baz.ini")
      .AddIni("whoops.ini", optional: true)
      .AddJson(source => {
          source.Path = "foo.json";
          source.Optional = true;
          source.ReloadOnChanged = false;
       }).AddXml(source => {
          source.Path = "bar.xml";
          source.Optional = true;
          source.ReloadOnChanged = false;
       }).Build();

cc @divega @shirhatti @lodejard