Updating CI Pipeline and build fixes (#9)
* Renamed and updated appveyor file (from yaml to yml).
* Changed nuget package format to snupkg so they can be uploaded to nuget symbol server.
* Added keywords to nuget packages as appropriate.
* Updated dotnet SDK version to use a wildcard for 3.1.*
* more build improvements
* Added support for SourceLink
* Build tweaks to:
* Update Appveyor build # with tag if present and only push to NuGet.org on tag
* Make non-tag master builds marked as prerelease
* Move version numbering to appveyor.yml so build number is consistent