Add --version

[guest]

cabal-install --version should return a version number.

[duncan]

We'll get this for free if cabal-install uses the Command abstraction, and --numeric-version and support for bash command line completion.

[duncan]

Now done.

$ cabal --help
Usage: cabal [GLOBAL FLAGS]
   or: cabal COMMAND [FLAGS]

Global flags:
 -h --help            Show this help text
 -V --version         Print version information
    --numeric-version Print just the version number

Commands:
  install      Installs a list of packages.
  info         Emit some info about dependency resolution
  list         List available packages on the server (cached).
  update       Updates list of known packages
  fetch        Downloads packages for later installation or study.
  configure    Prepare to build the package.
  build        Make this package ready for installation.
  copy         Copy the files into the install locations.
  haddock      Generate Haddock HTML documentation.
  clean        Clean up after a build.
  sdist        Generate a source distribution file (.tar.gz).
  hscolour     Generate HsColour colourised code, in HTML format.
  register     Register this package with the compiler.
  test         Run the test suite, if any (configure with UserHooks).

Typical step for installing Cabal packages:
  cabal install [PACKAGES]

Occasionally you need to update the list of available packages:
  cabal update

For more information about a command, try 'cabal COMMAND --help'.
This program is the command line interface to the Haskell Cabal Infrastructure.
See http://www.haskell.org/cabal/ for more information.

$ cabal --version
cabal-install version 0.4.1
using version 1.3.2 of the Cabal library

$ cabal --numeric-version
0.4.1