added imagemagick::config with $version

This is helpful for doing stuff like the following which allows the `rmagick` Ruby gem to be installed:

    file { "/opt/boxen/homebrew/Cellar/imagemagick/${$imagemagick::config::version}/lib/libMagickCore.dylib":
      ensure  => 'link',
      target  => 'libMagickCore-Q16.dylib',
      require => Package['boxen/brews/imagemagick'],
    }