Add a bunch of stuff

You didn't really list what you wanted done, so I just tacked on a bunch of stuff.

I'm assuming archive_dir is where you intend to keep info on what posts you have already submitted. I'd instead recommend storing the most recently submitted post's fullname as a message on reddit, and fetching that on startup. Keeps the bot's local environment simple. I've added a few functions that will store and fetch that info.

Removed loading the username from a file (not like it's going to change).
Added a meaningful user agent.