Add support for projects using PackageReference model

[ImrePyhvel]

Visual studio 2017 is supported and working as intended when the old packages.config model is used.

Problem occurs when user chooses the option to store package references in project file. Then the nuget package is installed, but the expected file of /Properties/Nortal.Utilities.AssemblyVersioning.MsBuildTask.props is not copied to project and build will fail. Example error message.
The imported project "W:\temp\ConsoleApp1\ConsoleApp3\Properties\Nortal.Utilities.AssemblyVersioning.MsBuildTask.props" was not found. Confirm that the path in the <Import> declaration is correct, and that the file exists on disk.

It should be investigated why nuget package content folder is not respected in this scenario.

The workaround is to manually add the missing file to /Properties/Nortal.Utilities.AssemblyVersioning.MsBuildTask.props from C:\Users\Imre\.nuget\packages\Nortal.Utilities.AssemblyVersioning.MsBuildTask\0.11.0\content\Properties\.

[ImrePyhvel]

PackageReference model no longer copies files from nuspec files element, but requires files to be set up in contentFiles section.

Also, the latter is intended for immutable files so not suitable for configuration scaffolding.

The intended way forward is to deprecate the props-file way of configuration and aim toward simplified configuration from csproj file itself. Especially considering the simplified SDK-style project files used in .net core projects.

[ImrePyhvel]

Solved as side-product of #12