Support for Custom Config sections for Azure Functions.

Currently there is no way to configure custom config sections for Azure Functions. Many third party libraries that we build components with require these custom config sections. Adding support for them would enable us to move many of our web service applications to Azure Functions. An example of a well known library that requires custom functions is the Google .NET DoubleClick Ad Client libraries at:

https://github.com/googleads/googleads-dotnet-lib/tree/master/examples/Dfp/CSharp

Thanks!

--Frank