Subreddit arg

Allow users to pick which subreddit to download from, wallpapers is nice but limited.
@dgary1980 Thanks for your PR :) we need to keep the /top.json segment intact though in this logic branch, mind fixing that? I.e wallj = Net::HTTP.get(URI.parse("http://api.reddit.com/r/#{@options[:subreddit]}/top.json?sort=top&t=#{@options[:top]}"))