Support `--prereq` CLI options

Since [the docs][1] mention it, and it's already supported via [config][2],
I thought I'd just pop the CLI support in.

This PR make `jshint` allow specifying the `--prereq` option on the command
line, accepting a comma-separated list of paths.

Test-related notes:

- Fixes the existing `testPrereq` test to check the right exit code. In its
  current incarnation, it could never fail (even when `lint` didn't pass)
- Added 2 tests for specifying the `--prereq` options from the command line.

Implementation notes:

I did the simplest thing possible (just merging the CLI `prereq` options in
with the config ones in the `run` method in `cli.js`). It's a little bit
awkward because there are 2 paths to `lint`:

1. `opts.useStdin`
2. `!opts.useStdin`

And each loads the config in a slightly different way. So the `mergeCLIPrereq`
function has to be in 2 code paths. So, slightly awkward, but not too terrible.

Fixes #1685.

[1]: http://jshint.com/docs/cli
[2]: https://github.com/jshint/jshint/blob/master/src/cli.js#L451-458
Any reason to not make this `null` to match other options around it? I know different options use `undefined`, `""`, and `false` somewhat inconsistently, but I think `null` is the right default here.