Add subgenerators

[sayedihashimi]

We should add the following sub-generators (item templates) for the initial release.

• New class
• New interface
• app.config
• AssemblyInfo.cs (called Assembly Information File in VS New Item dialog)
• Resource File
• xUnit test case

For the items above all should be equivalent to VS2015 New Item dialog. For xUnit we can base the content off their getting started guide at http://xunit.github.io/docs/getting-started.html#write-first-tests.

For info on how to implement the sub-generators you can see the generator-aspnet sub-generators at https://github.com/OmniSharp/generator-aspnet.

Ideally each PR will have mocha test cases that cover the additions. For examples on how to implement this see the tests in generator-aspnet https://github.com/OmniSharp/generator-aspnet/tree/master/test.