--hackage flag to attach to a specific hackage server

Hi Neil,
as promised, here's the first tentative patch.
The fix for CmdLine was trivial, so was the one for the action, because we had the CmdLine obj in scope.
A bit more problematic and unfortunate was to push to the bottom of the stack the HackageURL (type synonym created just to aid readability) when we need to create databases and thus items for them.
Last but not least, I was forced to expand "Convert" as well, because now the command makes sense only once we specify from which hackage we want to convert.
Thoughts? I wasn't sure of the code style either. Generally, whatever I've modified for the patch I've also tried to 
push it to <= 80 columns, please shout if it's not ok! :)

I haven't run the tests, but I did run "hoogle data" with and without the "-h" flag, observing that hoogle is now trying to fetch data according to the URL specified in the flag.

Thanks!
Alfredo