<Nul> separator for multi-line errors

[zenzike]

It seems that ghc-mod uses the <Nul> character to separate multiple line output. Sadly, this is problematic for the errorformat parser in vim (see vim-syntastic/syntastic#317). Perhaps it would be good if ghc-mod allowed this to be configured.

[kazu-yamamoto]

ghc-mod now provides "-b" command line option to specify a line separator string. I did not change its default value, ie a Nul string since a lot of confusion would happen.