Add myget.org as a package source

This allows us to consume forks of StyleCop.Analyzers and potentially specific versions of Roslyn if necessary.

nuget.config

@@ -9,6 +9,8 @@
   <packageSources>
     <clear/>
     <add key="api.nuget.org" value="https://api.nuget.org/v3/index.json" />
+    <add key="nuget.org" value="https://www.nuget.org/api/v2/" />
+    <add key="myget.org/F/aarnott" value="https://www.myget.org/F/aarnott/api/v2" />
     <add key="VSIDEProj (cpvsbuild)" value="\\cpvsbuild\drops\vsideproj\Packages" />
     <add key="VSPackages" value="\\airstore\dd\nuget" />
   </packageSources>