dotnet-new csproj templates (#4382)

Make csproj templates first-class in dotnet-new.

TestAssets/TestProjects/ProjectJsonConsoleTemplate/Program.cs

@@ -0,0 +1,9 @@
+using System;
+
+class Program
+{
+    static void Main(string[] args)
+    {
+        Console.WriteLine("Hello World!");
+    }
+}

TestAssets/TestProjects/ProjectJsonConsoleTemplate/project.json

@@ -0,0 +1,19 @@
+{ 
+  "version": "1.0.0-*", 
+  "buildOptions": { 
+    "debugType": "portable", 
+    "emitEntryPoint": true 
+  }, 
+  "dependencies": {}, 
+  "frameworks": { 
+    "netcoreapp1.0": { 
+      "dependencies": { 
+        "Microsoft.NETCore.App": { 
+          "type": "platform", 
+          "version": "1.0.1" 
+        } 
+      }, 
+      "imports": "dnxcore50" 
+    } 
+  } 
+}