Add support for loading from a .jshintrc file

I generally have my jshint options in a .jshintrc file to make it easy to run from the commanline tool (http://www.jshint.com/docs/). This is a change to allow you to set the jshint options to a path rather than having to define the jshint options in the gulpfile as well as in my jshint file. For example:
```js
var gulp = require('gulp');
var plato = require('gulp-plato');

gulp.task('default', function () {
	return gulp.src('app.js')
		.pipe(plato('report', {
			jshint: '.jshintrc',
			complexity: {
				trycatch: true
			}
		}));
});
```

This has helped me keep them in sync as I always have to just update the .jshintrc file