Add some user selectable prompt character options

This adds 2 new options:

'BULLETTRAIN_PROMPT_ROOT' - changes the default '$' prompt to an alternating '#' for root user and '$' for every other user.
BULLETTRAIN_PROMPT_CHAR - allows the user to specify a single character (e.g. '❯') to override the default '$' as their prompt character of choice. This is ignored if the length of the var/setting is greater than 1.

This also has the beginning of color changing options (e.g. red '#' for root, green '$' for normal users?) and could be expanded in the future for user settable color choices.