Add dotnet-watch to the templates

[empz]

Since we already have HMR for the client side source files it would be good if we also have dotnet-watch for the server ones so we don't need to stop and re-run dotnet run ever again during a development session.

[SteveSandersonMS]

Sounds great. Would definitely take a pull request for that :)

[empz]

@SteveSandersonMS I can submit a PR if you want but it's just a one liner.

Just add the package inside the tools section in project.json

"tools": {
  "Microsoft.DotNet.Watcher.Tools": "1.0.0-preview2-final"
},