Add options to turn off haskellmode's completion.

This patch gives users the option to turn off haskellmode's completion
since it currently tramples over the omnifunc (GHC_CompleteImports) and
completefunc (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 necomplete).