Migrate from licenseUrl to license since licenseUrl is deprecated and
fix grammar for VS version list.

Note: this chang also updates PUGIXML_VERSION macro to allow for
double-digit minor versions; this preserves the continuity of versions
so PUGIXML_VERSION >= 190 will still work.

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.

Also improve linkage description

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.