AsyncPackage for VS2015 and later

[duncansmart]

VS 2019 RC is showing this warning now:

The guidance is to move to the new AsyncPackage which was introduced in VS2015. This will make AutoT4MVC incompatible with VS2013 and earlier though.

I will happily create a PR for AsyncPackage - or do we want to maintain support for VS2013? Or do we want two versions to support both?

[bennor]

I don't think we really need to support below VS 2015 anymore. I would happily accept this PR.

[bennor]

Actually, thinking about it now, it could be a problem. I don't want people to not be able to install the old version, and I think that might happen if I push out a new version without support.

Any way round it?

Worst case I guess I could fork and add a new version for VS 2019.