urlselect: make secondary mode optional

- Disable secondary mode by dfault. You can set
  `plugins.var.lua.urlselect.enable_secondary_mode` to "yes" if you want
  to enable it.

- Remove /urlselect args that only available during selection from the
  help. And add 0-9 keys info to help.

- Add description for options using config_set_desc_plugin() and remove
  manual option descriptions from comment.

- Add URL into copied_url[] after calling external command