added (~>) infix operator alias for 'phony'

Discussion: https://github.com/ndmitchell/shake/issues/35

Main motivation is DSL consistency with normal rule `*>` operator.

**(Contrived) Usage:**

    -- Normal rule
    "article.pdf" *> \_ -> do
      need ["article.md"]
      cmd "pandoc" ["-o","article.pdf","article.md"]
    
    -- Phony rule
    "render"      ~> do
      need ["article.pdf"]
      cmd "echo" ["rendered."]

Choice of `(~>)` was more or less arbitrary