Integrate AddReferenceInFSI to VFPT

Close #897.

This is a faithful port of AddReferenceInFSI to VFPT.

I list here a few possible improvements for discussion:

 1. [x] The menu item only appears in `References` nodes in Solution Explorer. Should we display it when right click on Projects as well?

 2. [x] The feature generates references for all configurations in current project. Is this really necessary? Should we change to generate references for active configuration only?

 3. [x] A few known assemblies are generated using full paths (see below).  Perhaps we should use assembly names only?

    ```fsharp
#r @"C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.5\System.dll"
#r @"C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.5\System.Core.dll"
#r @"C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.5\System.Numerics.dll"
     ```
 4. [x] Hide the menu when the feature is disabled.

 5. [x] All `*.fs` files should be `#load`ed in single statement, otherwise types may not be available in subsequent files.


/cc @vasily-kirichenko @OkayX6 @halcwb