Use optLast instead of listToMaybe for sourceUrls/wikiUrls

This lets you override them using eg. cabal haddock --haddock-options,
which can come in handy if you want to use a different layout or URL for
your source code links than cabal-install generates.

This should partially address http://trac.haskell.org/haddock/ticket/79 since it lets you override Cabal's default locations with direct links to the line number.