Add support for relative paths

[Wackymax]

Hi, thank you for the amazing package.

I have the following project structure:

• MainApp
  -- lib
  --- l10n (symlink to -> ../../l10n)
• WatchApp
  -- lib
  --- l10n (symlink to -> ../../l10n)
• l10n

The symlinks are causing problems on some machines when cloned with git so I would like to specify the arb_dir parameter as "../l10n" but it's not working, it doesn't accept the relative path. Is this something that can be fixed?