Allow users to select the location of the wiki in the file system

Currently, we are hard-coding the location of the wiki pages in ~/.cristal.

But, it would be best to allow users to define where they want there wiki data to be stored.

We'd still need to keep an index of defined wikis, and in this case XDG (or the equivalent for other OSs) needs to be used to store the list of knows local wikis.

Note: we'll also need design for the create/add a wiki workflow.