Add another way to open the quickfix

It is possible to open the quickfix in another way by setting the
`g:ghcmod_open_quickfix_function`.
For example, it is effective in unite.vim and ctrlp and FuzzyFinder.
Tweak example
Simplify example of the quickfix
Supports autoload-function
Merge pull request #37 from shiena/patch/another_quickfix

Add another way to open the quickfix