Allow plugin to be added/removed without requiring a restart

[jjw24]

Allow plugin to be installed/uninstalled without needing to reload all via restarting the entire flow app.

DotNetCorePlugins, outstanding issue preventing usage

[taooceros]

Seems that .net core has supported dynamic assembly unload
https://docs.microsoft.com/en-us/dotnet/standard/assembly/unloadability