Fix the typo in config (directory -> default)
Update windows_safe_path to take an array of path pieces

This allows the user to specify a list of splat arguments to 
+windows_safe_path+ instead of just a string. Under the hood, 
+windows_safe_path+ will now use +File.join+ appropriately.
Project: +windows_safe_path+ expands paths now
Make sure Omnibus::Config extends Util

Prior to this commit, we were not using *our* +windows_safe_path+
method, and that's super awkward.
Remove Config.platformize_omnibus_path (no longer needed)
Update method signatures in Omnibus::Config for +windows_safe_path+
Update config specs to have more coverage