Bash completion for "cabal sandbox" subcommands

Command "cabal sandbox --list-options" doesn't list subcommands of
"cabal sandbox", but "cabal help sandbox" does. It turns out that it is
easy to parse its output using sed and as such can be added to list of
words for completion.
Merge pull request #2104 from trskop/master

Bash completion for "cabal sandbox" subcommands