Add `--load-character` argument to load a default character

[bmoconno]

I was looking for a way to start learning the repo, saw issue #542 and thought it was a good idea.

I'm not a .py programmer, so do let me know if there are any issues. It's also my first time contributing to someone else's repo... so hopefully I'm following the correct procedure.

• Add a new --load-character argument to the server.py, that loads the specified character JSON if it exists.
• Make a new log_file_prefix function in chat.py to ensure that persistent log files are named consistently.
• Add logic to chat.py to save logs after they have been cleared, and generate logs on character load.