Chill Update

- Added a very easy difficulty setting named Chill. Double damage and almost no spawned ichor fields. While I don't think a lot of people will play at this difficulty, I still want people to have the option.
- You can now change your difficulty at practically any time from the options menu. A reload from your last save or checkpoint is required in that case, however.
- Added a very small accessibility feature that allows you to darken the subtitle textbox background to make the text stand out a little more.