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.
Merge commit '6f7561eeb1fcff3c0f097744bb3840229606cbbb'

* commit '6f7561eeb1fcff3c0f097744bb3840229606cbbb':
  Add user prompt character options
Updates README