Add options to turn off haskellmode's completion.

This patch adds options for turning off haskellmode's completion since it currently tramples over the omnifunc (with GHC_CompleteImports) and completefunc (with CompleteHaddock), regardless of whether it is already set. This is a problem if the user wants to set omnifunc/completefunc to something else (e.g. if the user is using necoghc or an autocompletion system like YCM or neocomplete). The names of the added options are `g:haskellmode_completion_ghc` and `g:haskellmode_completion_haddock`.