Add option to exclude paid apps

[jdmcmahan]

Hey, this is really good code. Thanks for sharing it with us.

I thought the --include_free=true function would let me queue only the free apps. I'm trying to queue as much of the free content as my lan attendees have subscribed to, so that when they start downloading them from my otherwise fast cache, it won't go out to the internet to download it.

That said, could you do something like --include_paid=false or --only_include_free=true so that we could fill the cache with all the free content our users use?

Thanks in advance for considering this feature request..

Originally posted by @billthecatt in #54 (comment)