Implement user's config as init file and add directory into package.path

Create the user's config init file if lite user's config directory does
not exists.

No longer use the awkward package.searchers but instead add user's
config dir at the end of package path.

data/user/init.lua

@@ -6,7 +6,7 @@ local config = require "core.config"
 local style = require "core.style"
 
 -- light theme:
--- require "user.colors.summer"
+-- require "colors.summer"
 
 -- key binding:
 -- keymap.add { ["ctrl+escape"] = "core:quit" }