Make indentation easily configurable

[tibbe]

My personal preference is to use 4 spaces for indentation so I have set up haskell-mode to use that. However, I'd prefer that the where keyword was only indented 2 spaces (to set it apart from the rest of the code) and code under the where keyword was only indented 2 spaces (to not waste too much horizontal space when using where.) How difficult would this be to implement. Here's and example of what I'm currently getting:

foo x = y
    continued from above
    where
        y = z

This is what I want:

foo x = y
    continued from above
  where
    y = z