Support optional flag arguments after positional arguments

Nomad's CLI has always required positional arguments come after all optional flags:

```
# Works
$ nomad run foo.nomad
$ nomad run -detach foo.nomad

# Does not work
$ nomad run foo.nomad -detach
This command takes one argument:
For additional help try 'nomad job run -help'
```

As this causes some ambiguities, such as if there were a file named `-detach`, we would also need to support `-- [positional arguments]` syntax at the end of commands:

```
# Desired
$ nomad run foo.nomad -detach
$ nomad run -- -detach # Runs the file named -detach
```

It should be noted that the current behavior follows Go and BSD style, and the reordering behavior follows GNU style. I think the usability improvement is worth breaking out of our existing style, but it's definitely up for debate.

Start at Nomad's CLI entry point and locate the argument-parsing code and tests for the run command. Compare the current BSD-style behavior with the requested GNU-style reordering and `--` handling, then confirm the desired semantics before changing anything; done means both example forms work without breaking existing positional-argument behavior.

Written by the indexing model from the issue text.