Subreddit Stats: add option to include only self posts in stats

The option is just another flag, --self-only. It will exclude posts that are not self posts from the stats. If both the --no-self and --self-only flags are on, execution will continue after a warning message (it will not process any posts).

Unfortunately, some stuff (i.e. some of my extra documentation) will have to be culled out, but that shouldn't be too hard to deal with.

Anyways, if you notice something wrong with what I did (I coded the stuff in a period of 30 mins, so it might be a little sloppy), feel free to tell me.