[Implement] Code generation improvements

[greazer]

See issue #76 for details

[JakeRadMSFT]

See #76 for Spec

[LittleLittleCloud]

Changes need to be done on CodeGen side ( check code gen spec for details)

• add class ConsumeModel in Model project and update Program.cs
• using different annotation head for CLI and ModelBuilder
• CLI : This is an auto-generated file by Microsoft ML.NET CLI (Command-Line Interface) tool.
• MB: This is an auto-generated file by Microsoft ML.NET Model Builder.
• target generated Model project to netStandard2.0
• update namespace in generated Model project
• update namespace to AppML.Model.
• move ConsumeModel.cs and other two files to the same level of Model project csproj file

[LittleLittleCloud]

Changes need to be done on MB side

after CodeGen changes on AutoML get checked in

• update the code in “Next Steps” page to call ConsumeModel
• see the additional code with syntactic code colorization which is consistent with the current theme.
• add reference to Model Project
• add telemetry