add WatchWithContext() funcs for various providers where possible

[nilsocket]

This pull request closes issue: #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.

[knadh]

Thanks for the PR @nilsocket. Apart from changes to file.go which are simple, I'm unable to comprehend the changes to appconfig, nats etc (as I've never used those providers myself). Since this PR refactors the constituents of the existing Watch() functions, I'm unsure how to go about making sure that existing Watch() functionality is unchanged.

[nilsocket]

I have modified Watch() in order to limit duplicate code, should I modfiy it?
So, existing Watch() methods aren't touched.

[knadh]

Duplicating so much code wouldn't make sense. Are you in a position to test out Read(), and Watch() for these providers?

[nilsocket]

Yep, but I need some time for that (mostly a month).