Add a pretty printer

[ndmitchell]

So we can write code that takes a ghc-prof, throws away zero entries or similar (perhaps driven by app specific rules), and spits it out again for visualisation in another tool (e.g. yours or profiteur).

[maoe]

Sounds like a good idea. I'm in the middle of a refactor to support the new +RTS -pj format. I think I'll give this a try after that.