Store config files according to platform defined app config location

[cjbassi]

First off just want to say I enjoy wpm a lot. The issue I'm opening here is that the .wpmrc and .wpm.csv files are stored in the home directory but they should be moved to ~/.config/wpm/ by default.

[cslarsen]

Thanks! Just wondering why. How widely supported is this? I am thinking outside of Linux systems (these days that would mostly be BSD, I guess).

[cjbassi]

1. Keeps the home directory clean
2. Stores config files in one place
3. Stores each app's config files in a separate folder

It's the de facto standard on Linux and the recommended way to store app files now (it hasn't always been a standard) and I think the BSDs use it too.

[cjbassi]

So I forgot XDG is only supported on Linux and BSD, while OSX has its own standardization. go-appdir has a good list of what the standardization is for each platform, including osx. And osx support would have to be added to #43.