Fix nested load config

Fix jinzhu#34

Don't display warning log in watch mode

Fix file name

Add Auto Reload Callback

Don't overwrite current configuration if failed to parse new one

Fix read config from env

Add go.mod

Merge pull request jinzhu#37 from reuben453/error_on_unmatched_keys_u…

…pstream_pr

Add support for using the ErrorOnUnmatchedKeys setting with json files