share the hakyll-journal development sandbox

Sharing the development sandbox can save compliation time and space.  Notes:

* We `add-source` as well so that hakyll-journal will be automatically
  re-installed as necessary, when the source is updated.

* Running `cabal install` will put the journal executable into the
  hakyll-journal sanbox bin directory.  I will likely end up using an
  ignored softlink into `dist` for testing.