Allow pioneer to be run with cmd line args alone.

Should fix https://github.com/mojotech/grunt-pioneer/issues/5

```bash
$ pioneer features --require=steps --require=widgets
```

``` bash
fs.js:684
  return binding.lstat(pathModule._makeLong(path));
                 ^
Error: ENOENT, no such file or directory '/src/project/--require'
    at Object.fs.lstatSync (fs.js:684:18)
    at Object.realpathSync (fs.js:1272:21)
    at Object.expandPathWithRegexp (/src/project/node_modules/pioneer/node_modules/cucumber/lib/cucumber/cli/argument_parser/path_expander.js:17:23)
    at /src/project/node_modules/pioneer/node_modules/cucumber/lib/cucumber/cli/argument_parser/path_expander.js:9:39
    at Array.forEach (native)
    at Object.expandPathsWithRegexp (/src/project/node_modules/pioneer/node_modules/cucumber/lib/cucumber/cli/argument_parser/path_expander.js:8:11)
    at Object.expandPaths (/src/project/node_modules/pioneer/node_modules/cucumber/lib/cucumber/cli/argument_parser/support_code_path_expander.js:7:38)
    at Object.getSupportCodeFilePaths (/src/project/node_modules/pioneer/node_modules/cucumber/lib/cucumber/cli/argument_parser.js:44:83)
    at Object.
```

``` bash
$ echo '{ "require": [ "steps" ] }' > pioneer.json
$ pioneer features --require=widgets
```

Works :thumbsup: