Added 'session - display attributes (sda)' command.

Description: (sda) displays session attributes. Session attributes are statal.
The command is meant for developer debugging.

Example:

    Red Example Score (2013)

        build (u)
        makers (k)
        materials (m)
        segments (g)
        setup (p)
        templates (t)
        stylesheets (y)

    > sda

    breadcrumb_stack: ['Red Example Score (2013)']
    current_materials_directory_path: '/Users/trevorbaca/Documents/scores/red_example_score/materials'
    current_materials_package_path: 'red_example_score.materials'
    current_score_directory_path: '/Users/trevorbaca/Documents/abjad/scoremanager/scorepackages/red_example_score'
    current_score_package_manager: ScorePackageManager('/Users/trevorbaca/Documents/abjad/scoremanager/scorepackages/red_example_score')
    current_score_package_path: 'scoremanager.scorepackages.red_example_score'
    current_segments_directory_path: '/Users/trevorbaca/Documents/scores/red_example_score/segments'
    current_segments_package_path: 'red_example_score.segments'
    dump_transcript: True
    hidden_menu_sections_are_hidden: True
    hide_next_redraw: False
    is_backtracking_locally: False
    is_backtracking_to_score: False
    is_backtracking_to_score_manager: False
    is_complete: False
    is_displayable: True
    is_in_score: True
    is_navigating_to_sibling_score: False
    is_quitting: False
    nonnumbered_menu_sections_are_hidden: False
    scores_to_show: 'active'
    session_once_had_user_input: True
    show_example_scores: True
    snake_case_current_score_name: 'red_example_score'
    transcribe_next_command: True
    use_current_user_input_values_as_default: False
    user_input_is_consumed: False

    Press return to continue.