feat(samples): add sample extension with templates

[CalvinAllen]

Parent issue: #11

Create a sample extension demonstrating template support.

Structure

samples/SampleExtensionWithTemplates/
  SampleExtensionWithTemplates.csproj
  source.extension.vsixmanifest
  SampleExtensionWithTemplatesPackage.cs
  ProjectTemplates/
    SampleProjectTemplate/
      SampleProjectTemplate.vstemplate
      SampleProject.csproj
      Class1.cs
  ItemTemplates/
    SampleItemTemplate/
      SampleItemTemplate.vstemplate
      SampleClass.cs