Fix failure when only `script` is declared

When you only declare `script` you get this failure.

```
TypeError: Cannot call method 'split' of undefined
    at module.exports (/Users/shane/Sites/gulp-nodemon/index.js:8:66)
    at Gulp.<anonymous> (/Users/shane/Sites/solidweld/Gulpfile.js:11:3)
    at module.exports (/Users/shane/Sites/solidweld/node_modules/gulp/node_modules/orchestrator/lib/runTask.js:33:7)
    at Gulp.Orchestrator._runTask (/Users/shane/Sites/solidweld/node_modules/gulp/node_modules/orchestrator/index.js:271:3)
    at Gulp.Orchestrator._runStep (/Users/shane/Sites/solidweld/node_modules/gulp/node_modules/orchestrator/index.js:212:10)
    at Gulp.Orchestrator.start (/Users/shane/Sites/solidweld/node_modules/gulp/node_modules/orchestrator/index.js:132:8)
    at Gulp.run (/Users/shane/Sites/solidweld/node_modules/gulp/index.js:21:14)
    at /usr/local/share/npm/lib/node_modules/gulp/bin/gulp.js:115:19
    at process._tickCallback (node.js:415:13)
    at Function.Module.runMain (module.js:499:11)
```

This fix will allow either `script` or `options` to truly be optional.