ignore Cabal sandbox directories by default

Cabal sandboxes are sandbox environments for Haskell packages,
containing compiled packages and a bunch of metadata that users of
ack are unlikely to want to search.  Therefore ignore .cabal-sandbox
directories by default.

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I sent the patch to ack-users@googlegroups.com as the manpage requests, but
it doesn't seems to have made it through yet.  I don't have a Google account so
hopefully it shows up...