add "browse by conditional phrase"

[danxoneil]

If we add more conditional phrases, it would be good to have some method of exposing all items flowing from that phrase. Given the way the site is structured, this could be done by showing the "conditional phrase slug" for each because clause from this sheet: https://docs.google.com/spreadsheet/ccc?key=0AofAD3dmbOYCdFlzZkZfaVhMbWJ6MUk0dXR0ejFuZXc#gid=0 and allowing users to click on a conditional phrase slug to see all.