Allow custom .NET Core config prefix

[jason-roberts]

Originally from @steventmayer in #121 :

"After playing with the newest build, I was thinking of the ability to set the ToggleConfigurationSetting through an alternate constructor when you create the AppSettingsProvider, allowing you to set prefix that way. Another option would be to allow the user to set a configuration key through a custom attribute. This would allow flexibility in the different Configuration frameworks. I'll try out some options if I can get time."

[jason-roberts]

@steventmayer - unless you think this is urgent for 4.0.0 RTM we can push this to future minor release?

[steventmayer]

That sounds great to me. I am using the current prefix/config setup, and it is working well for me. If it gets 'unruly', I can also bring it to a separate configuration file. Thank you.