Allow omitting the double dash when running `cabal run`

As discussed in #1088, `cabal run` currently requires separating the executable's options from `run` flags with `--`:

```
$ cabal run foo -- -bar baz
./dist/build/foo/foo -bar baz
$ cabal run foo -bar baz
Unrecognized option: -bar
```

It would be nice to make the `--` optional while still allowing passing options (e.g. `--env-var`) to `run` itself. One way to implement this is to treat all input after and including the first unrecognised option as executable's options. However, making this work seems to require modifying both `Distribution.Simple.Command` and `Distribution.GetOpt`.

Start by reading Distribution.Simple.Command and Distribution.GetOpt, the two parsing areas named in the issue, and compare how cabal run handles options before and after --. Done means executable options such as -bar and --env-var are correctly distinguished without requiring --, while explicit -- continues to work.

Written by the indexing model from the issue text.