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