Update _cabal completion
Update _cabal completion

Adds support for cabal-install-1.20 features:

  * new `cabal freeze` command

  * new `cabal exec` command

  * new options (e.g. `--allow-newer`)

joshuarubin/dotfiles,joshuarubin/dotfiles,joshuarubin/dotfiles,joshuarubin/dotfiles