Add (optional) data persistence

Since the bot is scraping /r/all anyway, would be nice to build a dataset to play with later. The bot should store select information from all the comments it scrapes in a SQLite database. Also, the bot should store information about itself: in particular, what video links it's collecting from each subreddit. Would be interesting to see which videos are popular in which subreddits. Not sure whether or not the deduplication is something I care about or not here.

Data persistence should be turned on via command line argument: default bot operation should be as lightweight as possible.