Use storage_dir key in config for default storage directory

[mayppong]

Allow users to specify default directory in the config file for arc.

[igorbelo]

It's unnecessary since you can override the storage_dir/2 function in your definition file.

[pedep]

While i agree with @igorbelo, that you most likely will need to overwrite storage_dir in the individual definition, i see no reason we can't merge this. Are there any drawbacks?
Since its only a 1 line change, i doubt this will increase complexity. So unless a change like this will impact performance noticably, it LGTM.