Added custom directories to the search path

Hi,
I added the `--add-paths` option to add custom directories to the ".desktop" files search path. It allows user to specify directories via multiple `--add-paths` invocations (like `j4-dmenu-desktop --add-paths /first/path --add-paths /second/path`) or via a single one (`j4-dmenu-desktop --add-paths /first/path:/second/path`).

Please review the change and decide whether you'd like it in upstream or not. :)