add WatchWithContext() funcs for various providers where possible

[nilsocket]

This pull request closes issue: https://github.com/knadh/koanf/issues/340

Add WatchWithContext() func for following providers:

• appconfig
• consul
• etcd
• file
• nats

Other modifications:

• In file package calls to filepath.Clean() are removed as filepath.EvalSymlinks() will call Clean() on return.
• Helper function, inputFromConfig() is written and called along with Provider... funcs. As it's possible to call Watch() and then Load() which would cause issues.