Rely on mpv to know where to store application data & cache

This is the most platform-independent way, and fixes things
for users who have a global scripts directory. (see po5#66)

Closes po5#17

sponsorblock.lua

@@ -92,11 +92,16 @@ end
 options.local_database = false
 
 local utils = require "mp.utils"
-scripts_dir = mp.find_config_file("scripts")
+local function expand_path(path)
+   return mp.command_native({"expand-path", path})
+end
+
+local sponsorblock = mp.find_config_file("scripts/sponsorblock_shared/sponsorblock.py")
+local uid_path = expand_path("~~state/sponsorblock.txt")
+local database_file = options.local_database and expand_path("~~cache/sponsorblock.db") or ""
+mp.msg.debug("uid_path: " .. uid_path)
+mp.msg.debug("database_file: " .. database_file)
 
-local sponsorblock = utils.join_path(scripts_dir, "sponsorblock_shared/sponsorblock.py")
-local uid_path = utils.join_path(scripts_dir, "sponsorblock_shared/sponsorblock.txt")
-local database_file = options.local_database and utils.join_path(scripts_dir, "sponsorblock_shared/sponsorblock.db") or ""
 local youtube_id = nil
 local ranges = {}
 local init = false