new definition of special paths

[Canop]

A special paths entry is defined by 3 parameters: show, list, and sum.
Each of them can be default, always, or never.

Example configuration:

special_paths: {
    "/media" : {
        list: "never"
        sum: "never"
    }
    "~/.config": { "show": "always" }
    "trav": {
        show: always
        list: "always",
        sum: "never"
    }
    "~/useless": { "show": "never" }
    "~/my-link-I-want-to-explore": { "list": "always" }
}

Fix #687
Fix #669