Support multiple data/config directories

[Ralith]

The XDG spec defines $XDG_DATA_DIRS and $XDG_CONFIG_DIRS as preference-ordered lists of read-only locations to search for data and configuration, in addition to the writeable $XDG_DATA_HOME and $XDG_CONFIG_HOME locations, which have the highest precedence. These should be exposed. See prior discussion on the original repo, which includes a partial concrete API proposal.

[nmccarty]

This enhancement was discussed in the most recent maintainers meeting. The consensus was that this should be supported with a new struct that mirrors the existing API, but instead has methods returning iterators.

This issue is also of personal importance to me, so I will be following up with a prototype/sketch implementation soon.