Preserves custom root URLs - / - in menus, bypassing untrailingslashit c...

This addresses #308.

It adds a URL helper method that selectively applies untrailingslashit on URLs unless they = '/'

FYI, I added another menu item to TimberMenuTest to test this.

I'm still not sure if there is a better way [to set a menu link to "home"]( https://github.com/jarednova/timber/issues/308#issuecomment-50341251) that works across different environments but this (setting a custom menu item of /) is often a use case for me, so hopefully this looks OK!