Added go.mod support, fixed a bug and made a simple addition

[karanveersp]

Hi,
This library is really cool and useful so I wanted to understand how it worked. While reading the source, I was able to make some general refactors such as:

• Adding go.mod support
• Replacing interface{} with any which is an alias to interface{}.
• Fixing a bug where err was being returned instead of innerErr.
• Adding a public function GetApplicationDirPath which returns the path to platform-dependent application configuration directory. This is useful if the user of the library wants to manipulate it and needs easy access to that parent folder of the config for deleting it, or backing it up somewhere, etc.

I updated the test to use GetApplicationDirPath to verify it works.