Make `/` navigate into folders

[rodarmor]

This is a great feature from Quicksilver that would be nice to have in Alfred. In Quicksilver, pressing `/` while a folder is the current result will navigate into that folder. This is easier and faster than having to press cmd + arrow down, or arrow right, because it can be done without having to reach down to the arrow keys. Additionally, `/` isn't used in many searches, so it's less likely to conflict with existing use. This could be an opt-in feature.