Implement new architecture of dotnet-ef and PowerShell commands for ASP.NET Core/.NET Core

[rowanmiller]

Some things to consider:

• Do we continue to just shell to dotnet-ef from PowerShell commands? If so, how do we get better output (Improve compatibility of dotnet-ef logging with other scripting tools such as powershell #4771).
• Can we use AppDomains for xproj (NuGet Commands: Use AppDomains on xproj #4773)

[rowanmiller]

Since GitHub doesn't support multiple assignees 😄... assigning to @bricelam to catch up with @natemcmaster and others when he is back in the office to discuss where have landed and where we go from here.

[natemcmaster]

Remarking for triage: this can wait until after RTM.