Make the project more idiomatic Go

The main theme of these commits is to make the project more like other Go projects.

* Since it's a tool and having a single Config struct & loader don't make a lot of sense on their own, the entire thing is moved into the main package. 
* Being in the package main, it doesn't make sense to export anything.
* Fixed some issues Go lint complains about.
* Updated README as go get is enough to fetch & install tools.