topic: list the number of changesets when --verbose is used Displaying more information in the topic list is useful, we start with the most obvious: number of non-obsolete changeset in the topic.

topic: add color for the active topic We reuse the same color that the one used for the active bookmark.

topic: add formatter support Using formatter to output the list of topic will add support for color and richer templater. We add some test for output control. We introduce a dedicated test to help with coming more complexe output.