Make indentation level configurable

Indentation modes should start using this to decide how much to
indent. Some crazy people indent their Haskell by four spaces, rather
than two. When I have to work in those projects, I have to
conform. The solution is to add a .dir-locals.el file like this:

    ((haskell-mode . ((haskell-indent-spaces . 4))))

To indicate that this directory contains Haskell code that should be
indented four spaces.