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.
Merge pull request #138 from mboes/configurable-backspace

Make it customizable whether <backspace> deletes indentation too.