Create visual studio projects that are vs2019 compliant.
* nuget_build.ps1 :
Introduce a new argument that will define how we implement the nuget
build. For now we accept 201{9,7.5.3} as possible argument values.

* pugixml_vs2019{,_static}.vcxproj :
Add two visual studio projects that build pugi with the latest SDK and
build tools

* appveyor.yml
- Add Visual Studio 2019 to build targets
- Add Visual Studio 201{9,3,5} to build_scripts. And call
  nuget_build.ps1 with a new argument.
- Add Visual Studio 2019 to the test_scripts.

Due to a typo in build script v141 binaries were built using VS2015
instead of VS2017.

Fixes #157.

Unify build paths in all MSBuild VS projects and extract common build
logic into functions.

Note that this change changes both VS2010 and VS2013 projects to have
more predictable output paths and fixed output file name (pugixml).

We'd like to build pugixml with both static & dynamic CRT and put it
all in one NuGet package.

CoApp sort of allows us to do this via dynamic/static pivots, but it
does not let us customize the names of the pivots and additionally has
some bugs with the project setup. Their project modifications are also
much more complicated - really, at this point we should do this
ourselves.

Create a simple native NuGet package with Linkage setting that picks the
right library, and package all libraries appropriately. Note that we use
the unified path syntax to make it simple to just get the right .lib
file from the toolset/platform/configuration/linkage combo.

Correctly check for error codes and don't run .bat file since it doesn't
work anyway (the variables it sets aren't accessible in PowerShell, and
the path to the script doesn't seem to be the same in VS2017).