allow generated files to be saved

Love the initial cut of this this package. wkhtmltopdf is blazingly fast compared to the PHP libraries.

However, currently the package's two triggers, download() and stream() require a browser to be present to receive the result. So we can't queue the generation, or save the results to email to the visitor, or work with the package in Artisan commands.

This PR adds a save() trigger to store the generated output at a specified location and uses the Illuminate File class so that any file-related exceptions are generated for us, making possible all the use cases in the paragraph above.