Add support for generating VS2017-compatible .vcproj files

[donno]

Like #7, this should just require a new ProjectTool.VS2017 constant that maps to appropriate MSBuild file version and .sln file version.

The following is from a freshly created Visual C++ Win32 project solution file (.sln)

Microsoft Visual Studio Solution File, Format Version 12.00
# Visual Studio 15
VisualStudioVersion = 15.0.26228.10
MinimumVisualStudioVersion = 10.0.40219.1

And from the vcxproj file the <Project ToolsVersion="15.0"> while the PlatformToolset is v141 while there is a <VCProjectVersion> of 15.0