Add way of disabling vim2hs indentation

[dkasak]

Please consider adding an option of disabling the indendation part of vim2hs. This will enable people who use another plugin for Haskell indendation to still easily use other parts of vim2hs.

[gilligan]

@dkasak just curious : what other plugins provide good indentation for haskell ?

[dkasak]

@gilligan Well, there's haskellFoldIndent and hasksyn.