sup color customization

sup color customization

- config file is located under $SUP_HOME/config.yaml and has the following
  structure

  :colors:
    :symbol_name:
      :fg: <color>
      :bg: <color>
      :attrs:
      - <attribute>

  <color> and <attribute> can take the standard values available in the curses
  environment.
  There may be multiple attributes, but they need not be present.
- if there is an error in the user provided config file a default value will
  be used (stored in the Colormap class)