Added Visual Studio 2019 testing

.appveyor.yml

@@ -12,6 +12,7 @@ image:
   - Visual Studio 2013
   - Visual Studio 2015
   - Visual Studio 2017
+  - Visual Studio 2019
 
 environment:
   matrix:
@@ -46,6 +47,9 @@ before_build:
       if ("$env:APPVEYOR_JOB_NAME" -match "Image: Visual Studio 2017") {
           $env:generator="Visual Studio 15 2017"
       }
+      if ("$env:APPVEYOR_JOB_NAME" -match "Image: Visual Studio 2019") {
+          $env:generator="Visual Studio 16 2019"
+      }
       if ($env:PLATFORM -eq "x64") {
           $env:generator="$env:generator Win64"
       }