Implement a store_alist_dir function

We currently have the following functions:

- store_dir() (For storing a string with shell expansion to a dir/file)
- store_str() (For storing a string)
- store_alist_str() (For storing a list of strings)

we are missing a store_alist_dir() which stores a list of shell
expanded dirs/files.

Fixes #209: Implement a store_alist_dir function