Comment out `haskell-process-type` setting

This is not a good default, as not everyone uses `cabal-dev` and it
doesn't work for `.cabal`-less sessions.
Fix docstring markup
Mark `haskell-session-maybe` as autoloaded

This is to make

    emacs -Q -L . -l examples/init.el

work again, as otherwise `haskell-mode-contextual-space` would miss
`haskell-session-maybe`.
Fix thinko in column arithmetic for `-ferror-spans` (re #151)
Add `-ferror-spans` as default for `haskell-process-args-ghci`

This finally resolves #151 as now all pieces (including #62) are in
place for proper `-ferror-spans` support.