feat: resolves #57 : add support for "arrangement" option in `plot_annotation` method

[janaab11]

def plot_annotation(..., arrangement="pack"):
    ...

When arrangement is:

• pack: follows default grouping, by timestamps only
• stack: groups segments by their label. Each label gets a distinct line

[hbredin]

Thanks! Will definitely have a look at this PR but with summer vacations coming up, I cannot promise any ETA...