Follow-up tasks of timeline plots

[toshihikoyanase]

Results of optuna.visualization.plot_timeline was added by #8.
I think the current plots lack some test cases as mentioned in #8 (review).

For example, the following trials are not included in the current plots

• RUNNING trials
• WAITING trials
• trials whose datetime_completes are earlier than datetime_starts

Please add them to existing/new timeline plots.