Add user prompt character options

Add two new user options for changing the final prompt character (default '$')
1) 'BULLETTRAIN_PROMPT_ROOT' - allow the user to choose if they want the final prompt character to change to '#' for root user.
2) 'BULLETTRAIN_PROMPT_CHAR' - allow the user to choose their own final prompt character (instead of '$') as long as it's length is a single character

This also changes the color of the prompt character, red for root, green for normal user, and could be expanded in the future for user selectable color choices.