haskell-indentation: indent "=" following data/type/newtype declarations.

Whereas before the only available indentation was:

data Foo
= A
| B

Now indentation proposes the following:

data Foo
    = A
    | B

i.e. offsets the "=" and what follows by haskell-indentation-left-offset.
Merge pull request #139 from mboes/data-indentation

haskell-indentation: indent "=" following data/type/newtype declarations.