[Hackage] #393: Allow --preference= flags for soft constraints

[Hackage]

#393: Allow --preference= flags for soft constraints
---------------------------------+------------------------------------------
  Reporter:  duncan              |        Owner:                   
      Type:  enhancement         |       Status:  new              
  Priority:  normal              |    Milestone:  cabal-install-0.6
 Component:  cabal-install tool  |      Version:  1.2.3.0          
  Severity:  normal              |     Keywords:                   
Difficulty:  easy (<4 hours)     |   Ghcversion:  6.8.3            
  Platform:                      |  
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 `cabal install` now uses soft preferences from a `preferred-versions` file
 in the hackage index. It would sometimes be useful to also be able to
 specify these on the command line.

 It shouldn't be too hard since the solver already accepts such soft-
 constraints.

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Ticket URL: <http://hackage.haskell.org/trac/hackage/ticket/393>
Hackage <http://haskell.org/cabal/>
Hackage: Cabal and related projects