Add a section of progress to the progressbar.

        The progress is captured by "count" and displayed as a fraction
        of the statusbar width proportional to this count over the total
        progress displayed. The progress will be displayed using the "symbol"
        character and the foreground and background colours and display style
        determined by the the "color", "on_color" and "attrs" parameters.
        These parameters work as the termcolor.colored function.

def add_progress(self, count, symbol='#',
                     color=None, on_color=None, attrs=None):
        """Add a section of progress to the progressbar.

        The progress is captured by "count" and displayed as a fraction
        of the statusbar width proportional to this count over the total
        progress displayed. The progress will be displayed using the "symbol"
        character and the foreground and background colours and display style
        determined by the the "color", "on_color" and "attrs" parameters.
        These parameters work as the termcolor.colored function.
        """
        chunk = _ProgressChunk(count, symbol, color, on_color, attrs)
        self._progress_chunks.append(chunk)