Add ability to disable social provider

[typerandom]

Currently all social providers are enabled by default and it's not possible to disable them by local configuration.

I.e: https://github.com/stormpath/stormpath-node-config/blob/master/lib/strategy/EnrichIntegrationFromRemoteConfigStrategy.js#L100

We should respect the local configuration, so that the provider is disabled (enabled === false) then we should go with that. Else default to enabled = true.