Make it customizable whether <backspace> deletes indentation too.

Adds a customization option to haskell-indentation-mode, to allow for
<backspace> to behave like haskell-indent-mode, as well as to allow
<delete> to only remove one character, rather than whitespace from
point.
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.
Align arrows in types under "::".

Fixes #1.
Merge pull request #140 from mboes/align-arrows

Align arrows in types under "::".
Merge pull request #139 from mboes/data-indentation

haskell-indentation: indent "=" following data/type/newtype declarations.
Merge pull request #138 from mboes/configurable-backspace

Make it customizable whether <backspace> deletes indentation too.