Add subdirectory option

This addresses issue #6, allowing users to specify a subdirectory in their downloads folder where their files will be placed.

I also added the double-quote to the list of invalid characters, because Windows will prevent these filenames from being saved. I hope you don't mind.

If you want to reject this PR and rewrite it, feel free. I have almost no experience with Angular or CoffeeScript, so I'm not totally comfortable with their corresponding idioms.

I've also not included builds of the project in this pull request. If you want me to build it and add another commit, let me know.