Update cabal completion

Added support for the new cabal-install-1.18 features:

  * `cabal repl`, `cabal run` and `cabal sandbox` commands are added

  * `cabal get` is an alternative name for the `cabal unpack` command

  * some commands received new options

Also some unrelated enhancements:

  * generic `--with-PROG`, `--PROG-options`, and `--PROG-option`
  are replaced with real options (e.g. `--with-gcc`, `--gcc-options`,
  `--gcc-option` and the like) where possible

  * `cabal install` completion supports both [hackage] cabal packages
  and local .cabal files as arguments now